Transcript

2.
PELLETING -- BEFORE THE DIE
A. DEFINITION
Pelleting can be generally deﬁned as an extru-
sion type thermoplastic molding operation in
which the ﬁnely reduced particles of the feed
ration are formed into a compact, easily handled,
pellet. It is thermoplastic in nature because the
proteins and sugars of most feed ingredients
become plastic when heated and diluted with
moisture. The molding portion of the opera-
tion occurs when this heated, moistened feed is
forced into a die, where it is molded into shape
and held together for a short time. It then exits as
an extruded product. Pressure for both molding
and extrusion comes from pellet mill rolls which
force the feed through the holes.
B. ADVANTAGES
There are many ﬁnancial advantages to a pel-
leted feed product. These advantages are:
1. The combination of moisture, heat and pres-
sure acting on natural starches in feed ingredi-
ents produces a degree of geletinization. This
enhances the binding qualities of the starch-con-
taining ingredients resulting in better pellet qual-
ity. This improved feed conversion advantage
is particularly evident in the Poultry Industry.
2. Pelleted feed prevents selective feeding on
favored ingredients in a formulation. Since all
ingredients have been molded together, the ani-
mal must eat a balanced formulation, minimizing
waste and improving feed conversion.
3. Pelleting prevents segregation of ingredi-
ents in handling or transit. With medicated feeds
and concentrates, this avoids disproportionate
concentrations of micro-ingredients and resultant
ill effects.
4. Pelleting has been shown to reduce molds in
feed, again increasing feed conversion.
5. Pelleting increases bulk density, particularly
on alfalfa, beet pulp, gluten feeds and other such
ﬁbrous products. On alfalfa pelleting, for instance,
one can increase the bulk density by a ratio of
approximately 2 to 1. Densiﬁcation is, of course,
dependent upon the characteristics of the product
being pelleted. In bagasse, a by-product of the
sugar extraction process, we see densiﬁcation
from 8 pounds per cubic foot to 32 pounds per
cubic foot. The advantages in storage and ship-
ping are self-evident: higher pay loads and re-
duced bin requirements.
6. Round, densiﬁed pellets have much better
handling characteristics, which simplify bulk
handling. Often it would be impractical to handle
ingredients in bins if they were not pelleted.
There are also instances where extremely free
ﬂowing ingredients will ﬂood out of bins. Pelleting
these produces a form which can be easily con-
trolled.
7. Feed in pelleted form reduces natural losses.
Feeding range cubes to cattle is an application of
this advantage. Wind losses from feed bunkers
can also be reduced by pellet usage.
C. THE CHALLENGE
There are many advantages to the pelleting
process, but it is also a costly process. This
brings us to one of our major considerations in
this particular paper; minimizing cost per ton of
pellets produced. A thorough understanding of
pelleting fundamentals enables one to minimize
inputs such as energy, allowing us to keep the
cost per ton down, thereby enabling the user to
take advantage of pelleted feed.
We will begin by looking at some of the most ba-
sic principles of the process and build on these.
Don’t look for pat answers in this discussion. It
is questionable if they exist, in view of the many
variables one faces daily in pellet production.
D. FUNDAMENTALS AND THEORY OF OP-
ERATION
Let us ﬁrst look at the critical area where feed is
converted into pellets to see how a pellet mill acts
on the feed.
The basic function of a pellet mill is to form a
pellet. This actually begins at the nip point be-
tween the die and the rolls. All other portions of
the process are really supporting activities to the
action occurring in this critical area. One must
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3.
take a very close look at this area to fully under-
stand why it is necessary to feed the pellet mill
evenly, condition properly, etc.
Plate #1 shows the pelleting chamber; in this
instance, a two-roll pellet mill.
Plate #2 shows a close up of one particular
roll assembly and its relationship to the die.
Deﬁnitions -- Reference Plate #2
Roller Assembly - This is simply a cylinder
idling on bearings in much the same manner as
the front wheel of a bicycle. The only driving force
acting on the roller assembly is the frictional turn-
ing force from the die acting through a very thin
mat of feed between the die and the roll.
Die - The die is the driven component utilizing
power from the pellet mill motor. The die is perfo-
rated with holes through which material ﬂows at
pellet density. Perforation diameter and die thick-
ness determine the ﬁnal pellet size and quality.
Feed - This is the material to be pelleted after it
has been conditioned for extrusion.
Work Area - Work area in the pelleting chamber
can be deﬁned as that area where we receive
the feed at its own density, compress it and
force it into the holes in the die. In reality, there
are two portions of the work area.
Compression Area - Here the feed is com-
pressed to near pellet density, forcing out en-
trained air, with forced alignment of particles in
intimate relationship with each other.
Extrusion Area - Here the feed has reached pel-
let density and is forced to ﬂow through the die
perforations.
Plate 1: How a Pellet Mill Works
HOW A PELLET MILL WORKS
• Incoming material ﬂows into the feeder and
(when conditioning is required) is delivered uni-
formly into the conditioner for the controlled addi-
tion of steam and/or liquids
• From the conditioner, the feed is discharged
over a permanent magnet and into a feed spout
leading to the pellet die. (1)
• Inter-elevator ﬂights in the die cover feed the
material evenly to each of the 2 rolls. (2)
• Feed distributor ﬂights (3) distribute the material
across the face of the die.
• Friction drive rolls (2) force the material through
holes in the dies as the die revolves.
• Cut-off knives (4) mounted on the swing cover
cut the pellets as they are extruded from the die.
• The pellets fall through the discharge opening in
the swing door.
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4.
Plate 2 : The Die and Roller Assembly
Pellet Mill Forces
In order to fully understand how a pellet mill
works, one must be aware of the forces and how
they are applied within the pelleting chamber. In
particular, one must look at the forces acting on
a wedge of feed at the nip point in the pellet
mill. This is the real heart of the process and is
illustrated on Plate #3.
There are three main forces to be considered in
this analysis:
Roll Force - The force from the roll acting on the
material. This is the force that compresses ma-
terial and extrudes it through the die holes.
Die Force - This is the force from the die that
resists the ﬂow of material through the holes.
This force is designed into the system to produce
the ﬂow resistance or back pressure that forces
individual feed particles together, where they
bond and form the pellet.
Slip Resisting Force - Finally, there is a fric-
tional force derived from material contact with
the die. This particular force keeps the material
from squirting along the face of the die in front
of the roll. This force is related to the pressure
exerted by the roll and the frictional characteris-
tics of the feed itself. This force is similar to that
which brings a car to a stop when the brakes are
applied.
External Factors
To better understand the process, one needs to
evaluate what happens when there are changes
in the different variables.
Feed Rate - Plate #4 demonstrates what hap-
pens when feed rate is doubled. Note ﬁrst that
the mat thickness doubles in front of the roll.
This means there is a greater portion of the
force from the roll tending to push the feed
ahead of, rather than down through the holes in
the die.
This force tends to skid the feed along the face
of the die and can cause a plug in a pellet mill.
The feed mat thickness can reach a point where
the roll simply cannot grab it and instead begins
to push the feed forward along the face of the die
rather than down through the holes. At this point
the roll ceases to turn and the whole pelleting
cavity ﬁlls up (plugs) with feed unless caught by
the operator or process controller.
Keeping this in mind, one can readily visualize
what happens when there is a surging feed rate
to the pellet mill. First there is a very thin mat of
material ahead of the roll which can be readily
grabbed; then suddenly we have a big surge of
feed in front of the roll which cannot be grasped,
so it begins to slip. At best, one has a very erratic
operation, producing wild swings in the amme-
ter which measures mill main motor demand; at
worst, the pellet mill will not pellet. Therefore, do
everything possible to provide an even rate of
feed into the pellet mill, minimizing this problem.
Feed Distribution - Since this slip phenomenon
applies to each individual roll in its relationship
with the die, the need for an equal amount of
feed to each roll is obvious. Pellet mill produc-
tion is thus limited by the single roll that gets
the greatest amount of feed. There is also the
challenge of obtaining equal feed distribution
across the face of the die. For example, if all the
feed is at the front of the die, the mat thickness
is too deep for the roll to accept material, limit-
ing production capacity. When feed distribution is
controlled properly, spreading material across the
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5.
entire die, production capacity of the pellet mill is
increased. There will always be some side slip-
page under the roll of the pellet mill, but there are
deﬁnite limits as to how self-compensating this
can be. Feed distribution is the most over-
looked, yet most signiﬁcant, factor in a pellet
mill operation.
Roll Setting - Since the roll is turned by fric-
tional contact with the die, it must be adjusted
down to a proper relationship with the die, or it
will not rotate. Roll setting is critical to a pellet
mill operation, and the rolls must be set on a
regular basis. The ﬂow of feed passing through
the die normally wears the die down, away from
contact with the roll.
Maintenance - Adjustment of bearing clear-
ances in the roll assemblies as well as the
main bearing can be a signiﬁcant factor. If
there are excessive clearances in the bearings,
the roll is free to shift about its rotational axis and
move away from the die face. This generates a
skipping action, producing erratic pellet mill op-
eration. Loose main bearings in a pellet mill also
disturb the die/roll relationship. One can peen the
die (cold work it) if the die comes in hard contact
with the roll.
Frictional Characteristics of the Feed - Here
one can use the illustration of an automobile tire.
If attempting to run in snow, the tire slips and we
get nowhere. If you add sand or ashes under the
tire to increase friction, you stop the slipping. The
characteristics of individual feed ingredients act
much the same between the roll and the die. If
one adds too much moisture, the material has
a tendency to become slippery beneath the roll,
disturbing the driving force which turns the roll.
Here again the slipping roll will begin to plow
material ahead of itself. This explains why a pel-
let mill slips when one gets too much moisture
in the ingredients or adds too much steam. The
wet feedstock simply becomes too slippery, los-
ing its ability to turn the roll. This also illustrates
why it is critical to distribute moisture very evenly
on the feed ingredients. Moisture ﬂuctuations in
the feed ingredients themselves can also change
frictional characteristics and the operation of
the pellet mill. For instance, one can also have
a feed that is too dry and it will not want to slip
through the holes in the die. Resistance to ﬂow
through the holes can be greater than the force
applied from the roll, thus the die will quit accept-
ing the feed and the cavity will ﬁll.
Finally, we must consider the ingredients them-
selves. They vary in their frictional character-
istics, so if there is segregation or inadequate
mixing, we can have shifts from low to higher
ﬂow resistance. Under this situation, one will see
ﬂuctuating power demands and reduced pelleting
rates.
The Die - There can be changes in the die it-
self. If a die becomes too corroded, the surface
roughens and resists ﬂow to the point where
the pellet mill cannot accept feed. One can also
have cold working of the die face (peening) from
too hard a roll setting, which partially closes the
die hole inlet which increases ﬂow resistance and
reduces pellet quality.
Rolls - The face of the roll itself can change,
which reduces the frictional characteristics. This
normally happens when the outside diameter of
the roll shell is worn away due to abrasion from
the feed particles. If the roll face doesn’t wear
evenly, it can no longer maintain proper rela-
tionship with the die, and so they produce er-
ratic operation. There are also signiﬁcant varia-
tions in the concentricity of various vendors
roll shells and dies. This out of round condition
can both cause mechanical damage and/or make
operation difﬁcult.
With these basic points in mind, let us now look
at the various components of a pelleting system
and how they relate to the process.
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7.
E. PELLETING SYSTEM -- EQUIPMENT AND
INSTALLATION
1. General
Plate #5 is an example of a typical ﬂow diagram
in a pelleting cost center. It illustrates how mash
feed from the work bin ﬂows into the feeder
conditioner where steam and liquids are added.
The conditioned mash then ﬂows into the pel-
leting chamber where the pellet is formed and
sent to the cooler. In the cooler, the hot, moist
pellet is cooled and dried by air movement as
ambient air is drawn through the cooler with a
fan. Any ﬁnes entrained in the cooling air are
separated at a dust collector and returned to the
pellet mill where they can be reprocessed. Cool
pellets also can be crumbled to produce ﬁner
particles for feeding small animals. In many
instances, the product is then passed through
a screening mechanism where ﬁnal separation
takes place. Acceptable product goes to a ﬁn-
ished feed bin, while ﬁnes are returned to the
pellet mill to be reprocessed.
One should always evaluate the complete
pelleting system whenever a problem arises.
Don't look just at the pellet mill. To effectively
analyze a system, one should always provide ac-
cess for sampling to check what is happening at
different portions of the process.
Analyzing a system, one must ﬁrst consider what
is coming to the bins over the pellet mill. Look for
consistency of product. Considerations here
would be such items as mixer capacity, where
mixer demand has resulted in mix times below
that of the manufacturer's minimum or there
is severe mixer wear. Such problems produce
concentrations of various ingredients going
to the pellet mill. The pellet mill will surge as it
reacts to these concentrations. At a time like this,
one may be able to see differences in color or
grind in the bin sight glass.
Inadequate or poorly designed mash handling
systems can also cause segregation after
mixing, but before pelleting.
2. The Bin
The supply bin structures over the pellet mill
will vary with each installation. The more com-
mon design is a set of supply bins mounted
over a common surge hopper going to the pel-
let mill feeder. The supply bin or bins must be
of adequate size to provide a continuous supply
of feedstock to the pellet mill. The sizing of the
supply bins should be coordinated with the mill
mixing system to ensure an efﬁcient overall op-
eration. Experience indicates a need for at least
two bins, each at least 1-1/2 to two times the
capacity of the batch mixer. A bin installation of
this type normally results in an efﬁcient operation,
both from the mixing and pelleting standpoint. A
good surge bin design is essential to the pellet-
ing operation. There must be a steady ﬂow of
mash to the pellet mill. If there is any bridging or
acceleration in ﬂow, the pellet mill will react.
This can also obviously affect the conditioning
process.
Plate 5
Flow Diagram - Pelleting Cost Center

8.
The bin mounted directly over the feed screw
should have at least two adjacent vertical
sides, and two of these sides should be at the
beginning of the feed screw, where the feed
screw picks up most of its load. This is where the
mash ﬂow should be the greatest.
The other two bin sides should have different
slopes to produce an internal shearing effect
in the feed ﬂowing down the sloping sides. This
tends to break up arching formations. It is sug-
gested one face should have a 60° slope to the
horizontal, the other a 70° slope to the hori-
zontal. This is shown in the attached Plate #6.
Consideration must be given to the proper return
of ﬁnes from the dust collector and sifter. The
ﬁnes return line should come in at the rear verti-
cal face of the supply bin as shown on Plate #6.
The rear portion of the bin should be bafﬂed to
give the returning ﬁnes priority and prevent build-
up of ﬁnes in the return line. An 8" ﬁnes return
line is an adequate size, as long as there are
not condensate problems in the pellet cooling
system which would wet the ﬁnes and prevent
free ﬂow.
Notice also the bafﬂing for ﬁnes at the top of
the bin. Should there be an excessive amount of
returning ﬁnes, this bafﬂe will give them prefer-
ence as they move down into the main mash bin.
The secondary advantage of this system is the
ability to collect ﬁnes at the end of a run. The
pellet mill should be shut down while the pellet
cooler and the rest of the system are emptying
out at the completion of a particular formulation.
These returning ﬁnes can be accumulated in the
bin over the pellet mill and run out quickly. Fines
Plate 6
Pellet Work Bin Design
can be better conditioned with this approach,
which avoids continuous running with a very
small ﬂow of ﬁnes, decreasing the potential of
peening the die.
The spout connecting the hopper to the pellet
mill feeder should have a reverse slope where it
enters into the feeder. This is particularly neces-
sary with poorly ﬂowing feeds, because it guar-
antees a smoother ﬂow into the screw, giving a
more consistent, even feed rate. It also minimizes
any action by the screw which would tend to force
the material back up into the bin.
Whenever possible, a manual slide gate be-
tween the feed bin and the inlet hopper should
be installed. This provides a means of cutting off
the feed in the hopper over the pellet mill, which
may be necessary for maintenance of the feeder
conditioner.
Finally, the bin and its inlet should be designed in
such a manner that it does not segregate ingredi-
ents.
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9.
Plate 7
Feed Screw & Conditioner
3. The Feeder
The feed screw is the throttle for the pellet mill,
controlling feed rate. The screw itself should be
either tapered or of a variable pitch design to
permit the feed to ﬂow uniformly out the entire
bin discharge area. The feed screw diameter and
pitch must be balanced to the required feed rate
to avoid a surging discharge from the screw. Nor-
mal operation of the screw should be above 100
RPM to minimize this surging.
The feed screw is driven from a variable speed
motor and should have a range of speeds to
handle both the slower start-up feed rates and
ﬁnal production rates of all feed formulas. Pay
careful attention to the position of the variable
speed motor controls. Controls for the pellet mill
should always be located where the operator can
see the pellet mill ammeter, as well as check the
condition of the mash coming to the die.
An ammeter is used to measure the load on the
main drive motor at any particular feed rate. One
monitors the pellet mill power demand, both to
prevent overload and to observe the stability of
the operation.
4. The Conditioner -- Plate #7
The conditioner is a blending mechanism for
steam or liquid additives to the feed. Its function
is comparable to the carburetor in your automo-
bile.
For sake of simplicity, this discussion will pertain
mainly to the more conventional feed conditioning
system. Such systems would provide condition-
ing time of up to 15 seconds. There are many
special feed conditioners for speciﬁc applications
which could provide retention times as long as 20
minutes.
The conventional conditioner consists of a
chamber with a rotating agitator to blend ad-
ditives into the feed. Attention must be given
paddle adjustments so there is a proper level
of feed in the conditioner, giving adequate time
and action for blending and absorption.
Agitator tip speed is adjusted to the products
being pelleted and the retention time required for
proper absorption. Generally when one is pellet-
ing light ﬂuffy materials (less than 20 pounds
per cubic foot), agitator tip speeds will run be-
tween 600 and 900 feet per minute. On higher
density feeds, agitator speeds can reach between
900 and 1200 feet per minute for best results.
The function of the agitator is to blend, not beat
the pelleting is steam. The function of the
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10.
agitator is to blend, not beat the additives into the
feed. Agitator speeds should be kept as low as
possible to minimize abrasion.
The normal additive for feed pelleting is steam.
Steam should be introduced into the condition-
ing chamber at the bottom rear, with paddles
adjusted to keep a good head of feed in this
area. This adjustment to a half full condition
forces the steam to ﬂow up through the product
for even distribution. The agitator movement
gives an even, continuous blend of steam into the
product as individual particles are exposed to the
steam atmosphere.
5. Steam Addition
An adequate, well-regulated supply of steam is
essential to any efﬁcient pelleting operation. A
poor steam system causes difﬁculty for the pel-
let mill operator and plant management, creating
problems in stability of operation, throughput,
pellet quality and cost. This is true with a manual
operator or an ultra-sophisticated process con-
troller.
In planning a steam supply system, there are
three major considerations: Steam Quantity,
Steam Pressure, and Steam Quality.
a. Steam Quantity
Steam quantity comes from a properly select-
ed boiler. It should be sized to supply not only
the pelleting system but any auxiliary require-
ments within the plant. Steam quantity require-
ments for pelleting can be determined by using
the following process:
1. Establish the maximum production rate of
the pellet mill.
2. Multiply this production rate by the maxi-
mum amount of moisture that the feed will ac-
cept. A safe estimate ﬁgure here would be 6%.
3. Divide this ﬁgure (lbs. of steam/hr.) by 34.5.
This is the amount of water evaporated in one
hour at 212° F, which equals one boiler horse-
power.
4. Divide the above result by .83 (an approxi-
mate correction factor for 100% make-up water at
50° F).
Example: 12 ton/hr. production of poultry
feed with 6% added steam, so;
BOILER HP = (12 * 2000) (6%) = 50
34.5 (.83)
To simplify the process, Plate #8 provides a quick
reference chart for steam requirements with vari-
ous steam percentages and feed tonnages. Do
not skimp on boiler capacity. It can signiﬁcantly
reduce your production.
b. Steam Pressure
High pressure boilers (60 PSI to 150 PSI) are
considered more desirable than low pressure
units operating between 10 and 15 PSI. Use of
high pressure allows smaller pipes and smaller
control valves and keeps down costs. On the
newer, larger capacity pellet mills, it can be very
difﬁcult to ﬁnd ﬂow control valves of adequate
size for low pressure conditioning. Thus, most
customers now utilize the higher pressure sys-
tems.
c. Steam Quality
Having provided the necessary quantity of steam,
we must now deliver the steam to the pellet mill
at constant pressure and free of condensate.
A properly designed steam system is essential
and must be included in any well-designed pel-
leting system. Plate #9 shows such a set up for a
process control system. There are many process
control systems for pellet mills that provide au-
tomatic valve operation to suit the process de-
mands. In this kind of operation, all steam system
components remain the same except that an
automatically controlled steam ﬂow valve is used.
Piping size for speciﬁc steam capacities is avail-
able from any good text book, and installation
should be made accordingly. Adequate insulation
is always necessary to minimize energy losses
and condensate surges.
9

11.
Plate 8
Pellet Mill Thruput vs Steam Requirements
A strainer is recommended to keep scale and
foreign material out of the metering system. A
pressure regulator is essential to smooth out
ﬂuctuations in pressure from the boiler, because
varying steam pressure causes ﬂuctuations in
the ﬂow of steam through the control valve. This
varies feed moisture going to the pellet mill, with
resultant difﬁculties. We recommend that the
pressure regulator be able to monitor both up-
stream and down-stream pressures to guarantee
a smooth operation. Installation of a ﬂow control
valve should be made with the operator in mind.
These steam controls are normally placed ad-
jacent to feed controls no matter whether it is a
manual or automatic control system.
Condensation in a steam system can cause
many problems. It is best to remove as much
condensate as possible before it gets to the
steam addition system. Steam lines going to the
conditioner should be taken off the top of the
main steam header. This avoids picking up con-
densate lying in the bottom of the main line. The
steam separator should be sized for adequate
capacity and provided with a trap to remove con-
densate. The condensate must be completely
eliminated from the steam system. Thus it
should not be dumped back into a pressurized
condensate return system, but rather fed into
an atmospheric condensate return system. This
approach avoids back-pressure surges which
could blow condensate back into the conditioning
chamber. Such surges will plug a pellet mill
instantly.
The ﬂow control valve meters the quantity of
steam going into the conditioning chamber and
must be selected with care. For instance, pneu-
matic valves deﬁnitely need dependable actua-
tors. The ﬂow control valve itself should have
a linear response. Thus a normal gate valve
would not be adequate in most instances. It is
characteristic of a gate valve that as one ap-
proaches the half open position, small changes
in the valve setting produce large variations
in steam ﬂow. This makes ﬁne adjustment dif-
ﬁcult or impossible.
Manual shut-off valves are recommended
to turn off the steam completely during week-
ends or extended periods of down time when
mainte¬nance is required.
It is always good practice to provide an auto-
matic steam cut-off interlocked into the pellet
mill control system to shut off steam automatically
whenever there is a stoppage. First and foremost,
this provides safety for the operator. Secondly,
it eliminates the erroneous addition of mois-
ture to the feed lying in the conditioner, with
the resultant sticky mess that must be cleaned
out before the pellet mill can be restarted.
In the illustrated steam system, there is no provi-
sion to remotely change steam pressure as the
operator goes from one formulation to another.
Conditioning of the feed normally takes place
at atmospheric pressure. In this situation, with
an adequately designed steam system, there
is no potential for signiﬁcant variation in operat-
ing characteristics of high versus low pressure
steam. This is because the BTU energy value
of the steam that heats the mash changes very
little; any standard steam handbook illustrates no
signiﬁcant difference in BTU value between
10 PSI and 100 PSI steam.
10

13.
Plate 10
Molasses Addition System
6. Molasses Addition
Whenever molasses is needed in a formulation,
it must be blended very evenly into the feed.
The best way to do this is to break the molasses
into very ﬁne droplets with steam and inject it
into the mash in the conditioning chamber. Also,
the heated molasses more quickly penetrates
the feed, giving better absorption. The attached
Plate #10 shows how a molasses addition system
would be piped for best performance.
The system shown is extremely simpliﬁed to best
illustrate the molasses injection concept. There
are many sophisticated systems now on the
market, as well as process controllers that auto-
matically proportion the molasses in relation to
the feed rate coming to the pellet mill, but it still
requires a means to blend the molasses into
the feed evenly.
7. Pellet Mill
The pellet mill must be sized properly to EFFI-
CIENTLY handle one’s pelleting requirements.
The following application factors need to be
determined before proper selection of a pellet mill
can be made.
a. Types of formulation or ingredients used.
b. Capacity requirements in tons.
c. Pellet quality requirements, i.e., pellet durability
index.
d. Product mix -- both required pellet diameter
and length of run.
12

14.
There are two major performance criteria to be
considered in selecting a pellet mill for a speciﬁc
application. These criteria are: Retention Time
in the die and Power Requirements. These are
interdependent, so the proper combination must
be selected for a minimum cost operation.
a. Retention time -- Individual ingredients re-
quire a speciﬁc amount of time in the die to bind
together and form a pellet of the quality the cus-
tomer requires. The die working area, deﬁned in
Plate #11, and die hole drilling pattern control the
retention time for this part of the process. Techni-
cal data developed over the last ten years has
clearly shown that power consumption drops
dramatically for most formulations as the die
area per applied horsepower is increased.
This is perhaps best demonstrated by Plate #12.
For an integrated pelleting application, a pellet
mill with 500 square inches of working area and
300 applied horsepower would produce approx-
imately 32 tons per hour of product. With 800
square inches of die working area, utilizing the
same horsepower, one could produce 45 tons
per hour. The larger die is deﬁnitely required for
an efﬁcient operation. The dairy pelleting illustra-
tion shows the same improvement with increased
die area.
b. Horsepower requirements -- The power re-
quired to form a pellet is determined by both the
ingredients in the formula and the pellet quality
needed. Higher pellet quality requires higher
power input. We will give speciﬁc details relat-
ing to ingredients further on. However, one term
should be deﬁned here, indicating the power
demands. This term is lbs./HP hour (pounds of
pellets produced by 1 HP in an hour). Most ra-
tions can be grouped into categories that give
reasonably consistent production rates per horse-
power input.
For example:
Formulations with high grain percentages such
as poultry feeds normally produce in the range of
200 to 400 pounds per horsepower hour for an
integrated operation.
Plate 11
Die Deﬁnitions
I.D. – inside diameter of the die. This is the most common
identifying factor for die size.
O. – overall width of the die. There are normally two die
widths for each die diameter.
W. – working width, measured between the two inside
edges of the die grooves.
Grooves – cut on the inside circumference of the die, into
which the outside edges of the roll extend. This provides re-
lief for the ends of the rolls so that the roll can be adjusted
downward as the die wears away.
Die Working Area – deﬁned as the area between the two
inside die grooves. This area is what is available for drilling
the holes through which the pellets extrude.
Complete feeds typical of 12 to 15% complete
dairy feeds normally pellet in the range of 120
to 160 pounds per horsepower hour.
13

15.
Plate 13 Horsepower vs Die Working Area
High protein supplements, concentrates or
ﬁbrous products such as alfalfa normally pellet in
the range of 80 to 120 pounds per horsepower
hour. Plate #13 shows the inter-relationship
between horsepower, die working area and pel-
let type. Your pellet mill vendor should be able
to review your speciﬁc applications for capacity,
formulation and pellet quality and then ﬁnalize the
pellet mill selection for you. Your own individual
experience with speciﬁc formulations should also
be part of the selection process, which must al-
ways include the pellet quality criteria.
Die Speed - One should always run the pellet
mill as fast as possible for the pellet size in
production. The reason for high die speeds is
evident in our discussion of mat thickness ahead
of the pellet mill roll. We know there is a limit
to the thickness of material a roll can accept
for any given formulation. The way to maximize
production rate within these physical limits is to
speed up the pellet mill. This produces a thin-
ner mat layer for a given volume of feed, thus
producing better stability, potential for higher
conditioning temperatures, etc.
There is a limit to this concept. This limit is the
amount of breakage from impact as the pel-
lets leave the die and hit the stationary pellet mill
door. One can reach a point where the higher
impact speed causes so many ﬁnes it actually
reduces effective pellet mill throughput.
Pellet diameter is a major factor in determin-
ing proper die speed. As a general rule, small
diameter pellets in the 1/8” through 1/4” diam-
eter run best at higher speeds. Experience has
shown a die surface speed of 2,000 ft./min. is
ideal in most instances. Here we have the die
speed for maximum productivity balanced against
breakage of pellets as they hit the stationary pel-
let mill door.
Cubes are another matter, particularly the 5/8”,
3/4” and larger cubes. Die speed is much more
critical, and surface speed should be limited to
1200-1300 ft./min. to produce quality cubes.
Obviously there are certain applications where a
feed mill is required to produce both small pellets
and cubes. In this speciﬁc instance, dual speed
pellet mills are available to change die speeds
based on pellet mill size. Such speeds can be
changed either with mechanical transmissions
where one shifts gears, or with frequency varia-
tion on the main drive motors.
The importance of die speed is clearly evident
in applications using such materials as new crop,
higher moisture corn. With high speed pellet
mills there are usually no signiﬁcant variations in
pelleting characteristics; yet people pelleting the
same product on the same machines with lower
die speeds observed operational difﬁculties,
reduced productivity and reduced quality. The
reason is simple: the slower speed pellet mill
has too thick a mat of feed in front of the roll,
causing the roll to slip, which limits both feed
volume and conditioning
MAIN DRIVE TYPE
Two types of main drives are available for pellet
mills: the V-belt drive and the direct-connected
gear-drive. Generally, the V-belt drive provides
the lower overall cost per ton and is used on ap-
plications where one uses a single die to produce
most formulations. The simplicity of the V-belt
design provides the best operation. Where versa-
tility is needed, such as varying pellet sizes from
pig starter through cubes, the gear drive concept
is more practical. Gear-driven pellet mills can
14

16.
effectively utilize mechanical transmissions to
shift die speed. They also have the capability of
a quick cartridge change when a different die is
required.
Main Drive Motor - The pellet mill main drive
motor should be selected to function within the
duty cycle of the speciﬁc application. The horse-
power required is determined through an analysis
of capacity requirements and the power demands
of the formulations. One may wish to consider
purchasing the motor with a 1.15 service factor
to cover the amperage swings of a heavy duty
application, so it will run continuously at the rated
load.
Motor speed must be selected to attain the re-
quired die speed.
NEMA-B starting characteristics are desirable to
produce the torque required to push through
the small wedge of feed beneath the rolls re-
maining after a plug-up. Both across-the-line
and reduced voltage starters have been and are
being successfully used for pelleting applications.
The starter type and its selection depend upon
the characteristics of the electric supply coming
to the feed mill. NOTE: Care must be taken in
setting up a reduced voltage starter; there
should be enough starting torque to break
loose a plug in the pellet mill.
All pellet mill motors should be equipped with
inherent thermal protection to prevent over-
heating of internals. Such devices give more
efﬁcient and thorough protection than the heaters
in the motor starter itself.
Roller Assemblies - There are three signiﬁ-
cant factors in roller assembly design:
1. Adequate bearing capacity -- to withstand
stresses in the pelleting operation
2. Proper roll surface -- for maximum traction
and wear
3. Proper seal design -- to keep dirt from the
bearings.
Four basic types of friction surfaces are avail-
able for roller assemblies today:
1. The Tungsten Carbide Roll Shell - A rough
surface composed of tungsten carbide particles
embedded in a weld matrix, this is the longest
wearing shell available to the industry today. It
has excellent abrasion-resisting characteristics
and medium to high traction capabilities. It re-
quires special care during roll adjustment and
cannot be set on the die face, or it will immedi-
ately peen the die.
2. Corrugated Roll Shells - This is one of the
more popular surfaces used today. There are
two types, an open end corrugation and a modi-
ﬁed version where the ends have been closed
to reduce side slippage. The greatest advantage
of this type of shell is traction to reduce slipping,
particularly on the soft, less abrasive formula-
tions.
3. Indented Roller Shell - This type of shell has
indentations drilled in the surface which ﬁll with
feed and produce a friction surface for traction.
This speciﬁc design seems to be losing favor in
the industry since it has less friction resistance
than that of a corrugated roll shell.
4. The Coin Slotted Roll Shell - This type of
shell has coin-shaped slots machined in the sur-
face to improve its traction characteristics. Both
the indented and the coin slotted shells have a
tendency to slip as they begin to wear.
Dies - The die is the heart of the pellet forming
operation. Many characteristics of the die can be
varied to get the desired results on a particular
formulation. Often one must review die character-
istics with the pellet mill supplier to ﬁnd a solution
to a speciﬁc problem. In order to discuss dies and
die performance effectively, one should ﬁrst know
the terminology for a die.
15

17.
Plate #14 illustrates the signiﬁcant parts of a pel-
let mill die. They are:
1. d = pellet diameter
2. L = effective thickness. This is the length or
thickness of the die actually performing work on
the material.
3. L/d = performance ratio. This term relates the
effective thickness of a die to the diameter of the
pellet. Each ingredient has a speciﬁc L/d ratio,
required for it to be formed into a ﬁrm pellet of
the requested quality. This ratio describes the die
resistance in the force diagram in the earlier part
of our discussion. An example of this would be as
follows:
a. Ground corn normally requires an
L/d ratio of 12. (This means that if you are mak-
ing a 1/4" diameter pellet of ground corn, you
need a die at least 3" thick to get a good ﬁrm pel-
let.)
b. Alfalfa would require an L/d ratio of 8
and limestone would require an L/d ratio of 4.
Since each ingredient requires a speciﬁc L/d ra-
tio, changes in formulation will require chang-
es in die characteristics. One cannot indiscrimi-
nately change formulation without changing pellet
quality. Besides providing a means of discussing
any particular ingredient and its relationship to
die requirements, this concept gives the ability
to scale up or down in pellet size and be sure of
having essentially the same quality and produc-
tion criteria.
4. T = Total Thickness. Note that this is the over-
all thickness of the die. In many instances the
overall thickness of the die must be greater than
the effective length because of stresses within
the die from the pelleting operation. The overall
thickness of the die is required to withstand
the structural stresses of the operation. The
thicker the die, the stronger it is. Normal die thick-
ness increments vary by 1/4” between 1-1/2 and
5” thick.
5. X = Counterbore Depth. This is the difference
between the total thickness and effective length
of the die. A die is counterbored by taking
Plate 14: Die Characteristics
a larger drill and drilling in from the outside of the
die, relieving the pressure of the die on the mate-
rial. Counterbores can be supplied either with a
tapered bottom (shown in the diagram) or with a
square bottom. The square bottom counterbore
is normally supplied on feed mill dies since it is
least expensive to manufacture and normal feed
rations have little tendency to expand as they
leave the working length of the die. In some spe-
cial feed milling and industrial applications, there
is excessive expansion of the material as the
pellet leaves the hole. A tapered counterbore is
effective in minimizing a material’s tendency to
hang up in the counterbore and eventually form a
pellet equal to the counterbore diameter. Certain
materials may also require a tapered counterbore
to gradually relieve the pressure of the material
as it exits the hole. This can improve pellet quality
for certain materials.
6. D = Inlet Diameter. The majority of the dies
produced have a tapered inlet to ease the ﬂow
of material into the hole. This taper also begins
to compress the material as it enters the hole,
thereby doing work on the material.
16

18.
7. Compression Ratio = D2-/d2 (A relationship
of inlet area to pellet cross-sectional area.) This is
simply an indication of how we squeeze down the
material as it enters into the pelleting hole. On
small pellets, the compression ratio is normally
1.56 to 1. Compression ratios can become much
more signiﬁcant on large pellets or cubes and can
approach 4 to 1.
8. = Inlet Angle. This is normally a 30° angle
on small hole dies and just eases the feed into
the hole. The die will eventually wear to its own
angle after it has been in production, so the taper
is normally supplied at just the start of the ﬂow
until the die begins to wear. In certain instances,
where operator control is difﬁcult, dies can be
counterbored differently to minimize the potential
for peening.
NOTE: These terms apply to any die, small hole
or large hole. Cube dies do vary from the usual
small hole die in the inlet area because one sim-
ply runs out of die thickness required to form the
material. Dies are not normally made over 5"
thick, so one needs an additional means of doing
work on the feed to make it form up properly. By
increasing the cube die compression ratio (mak-
ing the inlet bigger), one can do more work on the
material. Therefore compression ratio and inlet
angles on cube dies have much more signiﬁ-
cance than that on small hole dies.
Dies are manufactured in a variety of sizes to
meet speciﬁc applications. Shapes are generally
quite limited because of the machining costs to
generate an exotic shape.
Small hole dies run in sizes from 3/32" in diam-
eter, to 1/8", 10/64, 11/64, 12/64, 5/16 and 3/8".
Normal range cube size dies are 1/2", 5/8" and
3/4" in diameter. Beyond this size, one encoun-
ters severe physical limits in relation to pellet
quality. The hole pattern of a die can be varied
to improve productivity or increase abrasion-
resistant quantities. It also can be modiﬁed to add
strength.
The alloy of the die can be varied to produce
maximum life. A variety of stainless steel dies
are used in pelleting formulations carrying cor-
rosive ingredients. Heat treating the die brings
out speciﬁc properties and varies according to
speciﬁc application, depending on whether abra-
sion resistance or toughness would be a major
criterion.
9. Process Control for The Pellet Mill
Process Controllers for pellet mills certainly have
come of age during the last few years. The cost
justiﬁcations deﬁnitely look attractive and the
industry now seems comfortable with them.
For background information, process controllers
are not really new. One of the earliest known
automation attempts on a pellet mill was in
1959 by then Sprout-Waldron in the Central
Soya Plant at Harrisburg, PA. The question was
not whether the system worked; the question was
how well it worked and what were the resultant
cost structures. At that time, cost structures could
not support the investment; the major reason
being the slow response time in actuation mecha-
nisms then available. This particular system was
pneumatically actuated. Since then, there have
been signiﬁcant advances in all aspects of hard-
ware (AC variable speed motors, for example),
greatly simplifying the process. Advances in solid
state computers have enabled systems to handle
data more efﬁciently as well as improve response
time.
Many vendors offer process controllers, each
with its own performance claims. The problem
becomes a matter of selecting the speciﬁc unit
to meet the needs and cost justiﬁcations of your
particular application. At the early stages, such
a project can be difﬁcult until one has an over-
view of the functions available for consideration.
Vendor literature and personal observation of
functioning plant systems will generate the initial
background required. Having developed this gen-
eral background, review your speciﬁc operation
and establish a set of goals for the controller. An
initial decision is:
Will the controller simply be a single pellet mill,
production control mechanism to cut direct labor
and improve throughput, or will it be integrated
into a complete management system, thus requir-
ing interfacing with other computers?
17

19.
AVAILABLE FEATURES:
With this very basic decision in front of us, let us
look at some of the many pellet system process
control functions that are available.
a. Upstream and downstream interlocks; i.e.
full bin, full cooler, etc.
b. Process controller to control the mash feed
rate as a function of the pellet mill main motor
load.
c. Ramp rate - Ability to change the rate at which
one increases feed coming to the pellet mill at
start-up. This would be a preset function, varying
with the formula type.
d. Operate at feed and steam set points input
manually by an operator.
e. Feed rate, steam and liquid addition either
from manual set points or stored data points for
speciﬁc formulations.
f. Anti-plug features with automatic restart and
return to production. This feature senses the pel-
let mill rolls as they begin to slip and stops in-
coming feed quickly enough to prevent the entire
pelleting cavity from ﬁlling and thus plugging the
pellet mill. Various companies have different de-
signs for this function. The best way to evaluate
design effectiveness is to visit an installation and
observe the results when you throw half a bucket
of water into the feed spout with the pellet mill in
full production. If the process controller catches
the problem, clears itself and restarts the pellet
mill, then the anti-plug mechanism is effective.
There are deﬁnitely units capable of this perfor-
mance on the market today.
g. Control of hot sprayed fat at the die.
h. An optimization procedure to obtain the
maximum mash temperature as the feed dis-
charges from the conditioner.
i. Multiple pellet mill operation from one con-
troller.
j. Monitoring pellet temperature rise through
the die.
k. Collection and print-out of operation and
maintenance data.
l. Sorting and accumulation of the data or tie-
in to other computers for downloading and subse-
quent data analysis.
m. Control of upstream and downstream func-
tions for grinding and/or outloading.
n. Modem interface to communicate with the
control supplier for trouble-shooting purposes.
The question is not whether the above functions
are performed, but instead how well are they
performed. The majority of reported difﬁculties
involve hardware response time or hardware
failure. Continual improvements are being made,
although hardware itself continues to be one of
the major hurdles as this process control concept
develops.
MISCELLANEOUS AREAS OF CONSIDER-
ATION:
Beyond observing installations now using various
vendor process controllers, there should be some
concern given to additional areas, such as:
a. What type of computer system:
1. Centralized - this controls all functions
of a feed mill, including the pelleting process.
2. Distributed control system - different
functional areas of the feed mill are operated with
separate, independent process controllers tied
into a mainframe computer to monitor the entire
operation.
The advantage of Choice Number 2 - if a comput-
er goes down, only that particular portion of the
feed mill would cease to function automatically.
18

20.
b. What amount of manual control for produc-
tion back-up is required for the speciﬁc applica-
tion?
c. Can the process controller software be
modiﬁed quickly and easily as system changes
occur?
d. What type of power failure protection is
provided?
e. Is the hardware for the particular model
"state of the art"?
f. What experience does the vendor have?
g. Does the vendor have the ﬁnancial depth
to stand behind his product and be available
years from now?
h. What will be the typical feed batch size?
This can affect the speciﬁc controller function de-
sired. For example, a 2-ton batch may not permit
time for an optimization sequence. In this situa-
tion, the run may be more effectively made in a
preset mode.
PROCESS CONTROLLER MECHANICAL RE-
QUIREMENTS
A pellet mill process controller requires equal
(and usually better) mechanical pellet mill
conditions and support systems than one run
manually. Steam systems or liquid systems that
the operator can run manually with compensa-
tion, for instance at reduced rates, simply will not
permit a process controller to operate. There-
fore, any system cost evaluation must include
the ﬁnances to get the mechanicals in proper
condition. Finally, process controlled systems
place greater demands on Management to set
and maintain programs for full maintenance and
utilization of available features. Such programs,
both for operation and data evaluation, should be
prepared before initial operation. There are sig-
niﬁcant costs involved in the purchase of a con-
troller; the full advantages of such systems must
be utilized to justify the expense.
OPERATION
We have now reviewed the basic equipment and
system parameters. Now we must turn our at-
tention to the system operation. The goal in any
pelleting operation is to produce a pellet of
acceptable quality while maintaining an ac-
ceptable production rate at minimum cost.
Remember that increased pellet quality demands
will decrease the pellet mill throughput.
Many factors are involved in making a good pel-
let: material density, source of supply, ingre-
dient quality, protein content, temperature,
moisture, die speciﬁcations and pellet mill
operation. Since all these factors inﬂuence pellet
quality capacity, it is impossible to set down hard,
fast rules governing all phases of pelleting.
The very nature of the Feed Industry is such that
the major ingredients are by-products of other
processes. Thus one is subject to variations in
those speciﬁc processes. These variables have
tended to make pelleting more of an “art”
than a “science”, though signiﬁcant strides are
being made in the sophistication of this process,
bringing these variables under more control.
Formulation
One should ﬁrst understand how formulation
plays a role in pellet production and quality, and
must at all times remember the action taking
place at the nip of the roll.
All are well aware of least-cost formulations from
a computer, and it only makes common sense
that due to price or availability formulas will be
changed. This is where the operating man’s
challenge begins. One must ﬁrst do everything
possible to get proper pellet rate and quality with
the formulas presented. Only then, when all me-
chanical means have been exhausted, would one
consider asking for a formulation change.
Let us look at some of the ingredient factors that
will be important in a daily operation.
A. Bulk Density
One will observe changes in bulk density of of
ingredients as received. This is an indication of
change in the basic characteristics of the
19

21.
ingredient. Generally, reduction in bulk density
means an increase in ﬁber, with the resultant
material handling and feed distribution prob-
lems in the pelleting cavity. It also normally
increases power demands. Therefore one
would anticipate that as bulk density goes down,
capacity goes down. An example would be, for in-
stance, between the pelleting of corn and alfalfa.
Corn at approximately 40 lb./cubic foot would
pellet in the range of 200-250 pounds per horse-
power hour while alfalfa at 20 lb./cubic foot would
pellet in the range of 100 pounds per horsepower
hour.
B. Texture
This factor is involved in grinding ingredients
for pelleting. In many instances, ingredients are
received ﬁne enough to be used as is in the pel-
leting process. An example of these would be
soybean meal, midds and things of this nature.
There are also basic ingredients such as corn,
which deﬁnitely must be ground before the pel-
leting operation. Grind can affect the capac-
ity through the pellet mill. A hammer mill is
designed to efﬁciently grind ingredients while
the pellet mill is designed for efﬁciency in the
agglomeration process. Therefore, if the pellet
mill has to perform grinding on the face of the
die, productivity will go down and die wear
will increase. Also, remembering the action at
the nip of the roll, it is obvious that long ﬁber
products such as alfalfa will not ﬂow easily. They
can become trapped on the ﬂat metal portion of
the die face between two pellet holes and must
broken before they can ﬂow down through the
die. If one grinds an ingredient ﬁner, it will ﬂow
more easily into the hole, thereby reducing power
requirements. Finer grinding of the products
also makes it possible for them to nest more
closely together, creating the potential for bet-
ter pellet formation.
Medium or ﬁne ground materials also provide
greater surface area for moisture absorption
from steam. This results in better conditioning
because of the increased exposure to steam
results in more rapid chemical changes within the
particles. This improves pellet quality.
Some older work done at Kansas State Uni-
versity, showing limitations on ﬁneness of grind
versus bulk density, may help in understanding
how grind affects the pelleting process. The effect
of grinding can vary from ingredient to ingredient.
In the case of corn, the greatest bulk density
for pelleting is achieved when about 20% of
the corn is ﬁne ground and the remaining 80%
is a coarse grind. The small particles can ﬁll in
the void between the larger ones. The elimination
of voids between individual particles improves the
contact between surfaces, improves binding and
pellet quality.
There have also been tests to show that mixing a
number of ingredients and grinding them together
can lower capacity and the quality of the pellet
mill performance. A variation of grinds tends to
do a better job.
An example of a preferred grind, particularly for
small pellets, would be as follows:
100% - 8 Mesh
35% (maximum) + 25 mesh
Some companies use much more involved grind-
ing specs, but others simplify it, stating a ﬁne
grind for pelleting should consist of 100% -14
mesh. Though opinions vary on the exact grind
characteristics, all agree that a variety of par-
ticle sizes is advantageous.
Coarseness of grind also relates to the pellet
diameter. For instance, in making a small pellet
with a coarse grind, a situation may arise where
one corn particle could extend completely
across the cross section of the pellet itself. This
provides a natural breaking point in the pellet,
reducing the quality and increasing the ﬁnes gen-
erated in the following material handling systems.
One can also see fracture points, particularly
in cube operation, when one tries to pellet the
large chips coming from the screening process.
Not only do these large chips provide an unstable
operation when they return to the pelleting cham-
ber, they also reduce quality. Therefore, a chip
grinder should be used in cube production,
reducing the the chips to granules before they are
returned to the pellet mill.
20

22.
C. Source of Supply
In some situations, there has been no change
whatsoever in the formulation going into a pellet
mill; yet one sees wide variations in the pellet-
ability of the formula. These can be traced to the
source of supply of speciﬁc ingredients. The
following are examples:
Alfalfa grown in Nebraska in sandy soil is more
abrasive than that grown in the rich black soil of
Northern Ohio. Abrasiveness is related to two
factors. First, there will be more sand in Nebras-
ka, which will obviously wear a die more quickly.
Alfalfa grown in dry areas will normally contain
more ﬁber than those grown with sufﬁcient rain-
fall. The higher ﬁber content in alfalfa reduces
the capacity of the pellet mill and increases
the abrasiveness.
Corn can vary considerably in bound moisture
content, depending upon the area where it is
grown and the rainfall received. Also, there are
differences in new and old crop corn, as well
as differences in how the corn is dried. This re-
lates to starch structures within the corn. Improp-
er drying techniques can make the starches
much less acceptable to the conditioning
process in the pellet mill.
By-products such as corn gluten feed offer dif-
ferent challenges. This feed ingredient varies
widely from supplier to supplier. Corn product
manufacturers use different processes for ex-
traction. There are variations in drying methods,
in amounts of starches and sugars actually ex-
tracted from the corn, and also in the amounts
and types of by-products being returned from
the process. Sometimes these variations can be
readily seen, with one shipment being dark brown
in nature, while others are light yellow and ﬂaky.
D. Oil Content
There are variations in natural oil or fat content
of the ingredients we use. For instance, in sol-
vent extracted oil meal, one would normally see
about 1/2% or less residual fat while in some of
the older expeller type processing, one could
see 8% to 9% fat. Differences in lubricity and
ﬂow characteristics are signiﬁcant. The solvent
process is now being used in most operations to
extract more fat from the oil, so we must antici-
pate changing pellet characteristics for this type
of ingredient.
E. Added Fat
Addition of fat to a formulation should be done
with a careful eye toward the desired results. In
this instance we are talking particularly about
fat to be added before processing through the
die. Fat will always lubricate the ﬂow of mate-
rial through the die, reducing ﬂow resistance
or back-pressure and thus reducing the pellet
quality. There is a rule of thumb for competitive
situations where pellet quality is signiﬁcant: One
should limit fat addition to a maximum of 1/2 of
1% in the formulation coming to the die. Anything
beyond this is going to create quality problems.
To put it in everyday terms, you wouldn’t grease
a handful of marbles if you wanted to glue
them together.
Fat is used primarily in integrated feed manufac-
turing facilities, where ﬁnes may not be a signiﬁ-
cant problem. An annular gap expander should
be considered to pre-process feed before the
pellet mill, if both high pellet quality and high
fat are required
Some articles have been published indicating
advantages of having ﬁnes in the pellets be-
cause of increased conversion ratios. Some
do add 1/2 to 3% in formulations under these
conditions to make a pellet they consider accept-
able. Die thickness should be carefully reviewed
to give the proper L/d ratio for these production
situations. One of the approaches for fat addition
is to spray fat on the pellets as they emerge
from the die. The pellets are warm and readily
absorb the fat up to percentages approaching
4%. This minimal capital cost approach to fat ad-
dition is normally done on integrated operations
where pellet quality is not a signiﬁcant factor, but
has a potential of causing problems in the
downstream processes. Fat can accumulate
in pellet coolers and air systems, increasing
maintenance costs. Recent studies on pellet-
producing operations for a competitive market
indicate that the older approach of spraying fat on
the pellets after the cooler produces better pellet
quality. Data indicates that the more deeply
21

23.
absorbed fat from a spray on the die system will
reduce pellet durability and leave more ﬁnes in
the conveying troughs of the feed-out operation.
F. Fiber
Fiber can be a natural binding mechanism but is
unfortunately difﬁcult to compress and force
through the holes in the die. Usually a high ﬁber
feed produces a tough pellet that results in low
production rates per applied horsepower.
G. Protein Content
One would normally expect high production ca-
pacity with good natural protein ingredients. The
major contribution of protein is the fact that it will
plasticize under heat, even frictional heat as the
material passes through the die. This plasticity
aids in the formation of the pellet and the adhe-
sives bind the pellet together.
H. Urea Content
Addition of urea to formulations has the effect
of reducing pelleting rates and increasing die
costs. This is related to the amount of steam that
can be added to this ingredient without creating
hang-up problems in the bin.
I. Mineral Additions
Minerals such as limestone, di-cal and salt are
very tough to pellet and produce at low ca-
pacities. These types of products have extreme
resistance to ﬂow through the die, so a very thin
die is required to keep resistance under control.
Counterbored dies often are required to meet
the balance between high stress and minimum
thickness for pellet formation. In adding salt,
one must consider the corrosion factor that
can accelerate wear within the die.
J. Molasses
Molasses is used in many feeds because of its
carbohydrate value and its ability to increase
feed. It also remains a reasonably cheap com-
modity. Ruminant feeds contain fairly large levels
of molasses. Molasses can be premixed ahead
of the pellet mill, or it can be injected directly
into the conditioning chamber. The difﬁculty
encountered with mixing molasses before the
pellet mill is that it tends to plug up the bins if it
reaches an excess of 8 or 9%. There are also
problems with buildup on metering screws and
walls of conditioners when one uses premixed
molasses.
The amount of molasses that can be added to
a formulation depends upon absorption char-
acteristics. Low protein ingredients generally
can absorb more molasses than high protein.
The higher the moisture content of the ingre-
dients, the less molasses it will absorb. Cold
ingredients will cause molasses to congeal
on the outside and form balls. Molasses will
be absorbed much more readily if sprayed on
warm materials.
Molasses itself is quite a variable product. Com-
panies selling molasses have blending facilities
to reduce the variations and the difﬁculties it
causes. There are variations in the types of gums
as well as in caramelization temperatures, all of
which affect molasses’ addition to the pelleting
process. Molasses contains 20-25% water. This
affects the pelleting operation, because this water
limits the amount of steam one can apply in the
conditioner.
Ambient Conditions
Both temperature and the relative humidity to
which ingredients are exposed can affect pellet-
ability. Extremely cold winter conditions produce
lower mash temperatures coming to the pellet
mill. Northern installations routinely have prob-
lems reaching as high a mash temperature in the
winter as in the summer. One simply cannot add
enough steam to raise the temperature without
making the mash too wet to pellet. The section
on conditioning will further explain these limits.
Experience indicates that ingredients exposed
to high humidity can pick up moisture, affecting
their ability to be heated without becoming too
wet. There have been problems getting accurate
documentation on this fact, but data available
tends to support this theory.
Pellet Mill Operator
The operator should be conscientious, capable
and readily available to input the data required
for the operation, whether one is dealing with a
totally manual system or an automatic system.
22

24.
The system should also be designed so that
the operator can see the ﬁnished product and
evaluate the performance of the pellet mill vs.
the operational settings.
Conditioning
Assuming proper equipment selection and instal-
lation provides an even ﬂow of mash to the pellet
mill, steam then becomes a major factor in the
pellet mill operation, since it lubricates, soft-
ens, and can improve the binding characteris-
tics of materials being pelleted.
First we must understand the two conditions un-
der which moisture is present in the feed going to
the pellet mill.
a. Bound Moisture - this is the moisture within
an ingredient as received. It can vary with the
source of supply and the manner in which the
ingredient has been handled.
b. Added Moisture - This is the moisture added
at the conditioning chamber, principally for lubri-
cation. In this instance, one is attempting to coat
each particle of feed with moisture while heating
it. This enables the material to slip through the
die easier, reducing frictional heat and increasing
die life. The added moisture also dilutes natural
adhesives in the ingredient and begins chemical
changes that will assist in better pellet quality.
The moisture is added as steam which condens-
es on the individual feed particles giving up both
heat and moisture. Experience indicates that the
maximum moisture we should anticipate add-
ed in the conventional conditioner is 6%. A
conventional conditioner might be best described
as one having between 12 and 18 seconds reten-
tion time in the conditioning chamber. Beyond
this range, most materials become too slippery to
be trapped by the roll and forced through the die.
Also, beyond 6% addition and with limited reten-
tion time, natural adhesives become too diluted
which reduces pellet quality. The steam condi-
tioning process should be evaluated within these
parameters for normal, conventional conditioning.
The next step would, of course, be additional
conditioning time in the 2 to 20 minute range to
permit additional absorption into the ingredient
itself. One must always remember when add-
ing moisture that there must be allowance for
its subsequent removal in the cooling pro-
cess, or the pellets can mold and spoil.
Advantages of Steam Addition
a. Increased Production - Plate #15 shows the
relationship between steam ﬂow and produc-
tion rate. This particular installation was a turkey
formulation. While exact numbers may vary from
one formulation to another, the effect is as il-
lustrated. There have been many documented
experiments in which production rate increased
over 300% as steam softened ﬁber and lubricated
ingredients to ﬂow through the die.
b. Increased Die Life - Plate #16 ﬁrst illustrates
the situation where the operator adds steam to
bring the mash temperature to 120° F. With the
pellet mill running at full load, the temperature of
the pellets leaving the die was 160° F. This is a
40° F. temperature rise by frictional heat as the
mash is forced through the die. This increased
temperature represents additional wear on the
die. As the operator opened the ﬂow control valve
to heat the meal to 175° F. and increased the pro-
duction rate to the pellet mill, the pellets reached
Plate 15
Production vs Steam Flow
23

25.
180° F. leaving the die. This 5° temperature gain
represents a 3% frictional heat pick-up. Heat
gain is directly related to die wear.
Plate 16
Die Life vs Conditioning Temp
Plate 17
Power Demand vs Conditioning
c. Power Reduction - One can readily demon-
strate the effects of steam on power reduction
in the pellet mill. Plate #17 indicates the savings
possible with the proper use of steam. This par-
ticular test reduced electrical power require-
ments approximately 600%.
d. Improved Pellet Quality - Plate #18 clearly
indicates a relationship between ﬁnes and
steam ﬂow rate. As the steam control valve was
opened, ﬁne percentage went down until the
choke point was reached. Note that the ﬁnes
rate was cut almost in half. Such comparisons
must always be based on a pellet mill with proper
die selection.
The thermometer on the pellet mill can only
indicate the temperature of the mash. It does
not tell what temperature can be run with a par-
ticular formulation for the best quality. This must
be checked as the pellet mill is challenged to get
the very best conditioning temperature. There are
two time-accepted methods of checking physi-
cally to get a good indication of potential quality.
Take a few pellets just as they come from the
pellet mill and roll them between your ﬁngers
to check whether you have softened the natu-
ral adhesives and achieved the plasticity re-
quired. If the pellets immediately break up and
go back to ﬁnes as they are being squeezed,
they have just burnt together on the outside.
However, if they remain soft and plastic, one
has come close to optimum conditioning.
Another means of testing, where temperature and
safety permit, is to take a handful of hot mash
from the end of the conditioner. Take a pinch
between the thumb and index ﬁnger and make
a wafer approximately the size of a quarter. If
this soft plastic wafer can be moved back and
forth through the air in a horizontal position
without breaking, one has done a good condi-
tioning job. There are optimum conditioning tem-
peratures for different types of ingredients --- the
following lists ﬁve categories in which the major-
ity of formulations fall. These should be used for
guidelines as one challenges the pellet mill.
24

26.
Plate 18
Finves vs Conditioning
Types of Feeds
Category I - Heat Sensitive Feeds
These feeds contain 5 to 25% sugars, and/or
dry milk powder or whey. These heat sensitive
materials will begin to caramelize at about 140°
F. As caramelization begins, the product tends to
stick to the holes in the die, further increasing re-
sistance. This can build in a chain reaction until it
shuts down the pellet mill. If a relatively thick die
is used, without lubrication, natural frictional heat
can raise the temperatures above this point.
One corrective action is to use a very thin die,
thereby cutting down the work one performs on
the material. This was generally uneconomical in
the past because of the length of time required to
change dies. With the advent of the cartridge-
type, quick change pellet mill, the die change
becomes more feasible. Whether one can
afford to change the die remains the limit. How-
ever, if a large percentage of the formulations
has these characteristics, the cartridge concept is
justiﬁed.
If only a small percentage of the total production
is heat sensitive, other corrective action may be
taken. In some instances, it is practical to add
fat to provide the lubrication required to ease
the product through the die without raising tem-
perature. It may be an expensive ingredient, but
when one considers the potential down time of
a plugging pellet mill, fat begins to show its ad-
vantages. Too much fat can be added, which can
reduce the quality of the pellet beyond the point
of acceptance.
Addition of water as a solution to the problem
has also been suggested. This gives sufﬁcient
lubrication to permit passage through the die
without reaching the critical point of 140° F.
There are very deﬁnite limits to this option. While
it is possible to increase production, one can
produce sticky pellets that will plug coolers, etc.
Attention must also be given to spoilage, since
too much moisture can cause spoilage in the
bin.
Category II - Complete Dairy Feeds
Complete dairy feeds (12 to 16% protein) gener-
ally must be treated separately because they ﬁt
none of the other categories. These formulations
are neither high in grain nor protein and contain
a fairly high percentage of light, ﬂuffy rough-
age ingredients. This combination lowers the
ability of the mix to accept moisture. Usually a
percentage of molasses is included in this type
of formulation. The moisture from the molasses
further restricts the addition of steam to the mix.
Generally speaking, mash moisture going to the
die should be in the range of 12 to 13%. This
means that temperatures will normally be held at
130-160° F. Steam addition to raise moisture
and temperatures higher than this generally
results in quality deterioration, as it dilutes
adhesives in the formulation and lets the pellets
expand and crack immediately after leaving the
die. Quality is a signiﬁcant competitive factor on
this type of formulation, and poor pellets cannot
be tolerated.
Category III - High Natural Protein Feeds
This category includes natural protein contents
between 25 and 45%. It also contains 5 to 30%
molasses. Some dairy feed, steer feed supple-
ments or concentrates normally fall in this cat-
egory. As such, these formulations require a great
deal of heat but not as much moisture as the high
25

27.
starch feeds. These will gum and choke the die
at much lower moisture levels. 1 to 2% moisture
may be added for lubrication, but heat is the
main demand.
These feeds are particularly difﬁcult to run
during cold weather conditions where we are
dealing with low mash temperatures. There
can be instances where it is not possible to
get anywhere near the needed temperature,
and one only has the frictional heat of the die.
Extended conditioning time to permit liquid
absorption has proved to be a beneﬁt with
this type formulation.
Category IV - Starch Feeds
These are complete feeds with high grain
percentages (50 to 80%) and protein running
under 25%. The key factor to remember in pro-
cessing this type of feed is gelatinization. In the
feed pelleting sense, gelatinization could be de-
ﬁned as a complete rupture of the starch granule,
permitting it to act as a binder. Thus gelatinization
is a breakdown of starches into simple sugars.
When the pellets cool, the sugar serves as a
binder. Total gelatinization is not achieved, and
studies indicate that only about 16 to 25% total
gelatinization can take place in these conditions.
There are three factors involved in the gelatini-
zation process; time, temperature and mois-
ture. The addition of pressure and mechanical
shear accelerate the gelatinization process and
these mechanisms are deﬁnitely available via
the pellet mill. We need both high heat and high
moisture to get good quality. Total mash mois-
ture can be brought up to between 16-17 1/2%
before reaching the plug point on the die. In
this instance, one deﬁnitely does not want liquids
added before the pellet mill. Instead, one should
put just as much steam as possible on the mash
to bring moisture and temperature up in a proper
relationship. The temperature must reach at
least 180° F to achieve good binding charac-
teristics. In this formulation, problems encoun-
tered usually are in product quality, not pelleting
capacity.
The recommended level of temperature/moisture
for pelleting these high starch formulations has
been determined through a series of controlled
experiments. In one test the temperature was
held constant and the moisture was varied. In
the next instance, the moisture was held con-
stant and the temperature was varied. Finally,
the third test was conducted varying both. The
test indicates the best results were achieved
with moisture at 16 to 17% with temperatures
above 180° F.
These types of formulations run into difﬁculties
with low mash temperatures in the winter.
With very cold ingredients, one can add steam
and reach a choke point from the moisture stand-
point before reaching the temperature required to
gelatinize. Quality suffers automatically.
There is a rule of thumb used in the pelleting pro-
cess; for every 20° F. temperature rise of the
mash when adding steam, add 1% moisture
to the product. The speciﬁc number can vary
signiﬁcantly, both due to ingredient type and/or
bound moisture of the ingredients. but the rela-
tionship exists.
Plate #19 clearly shows the relationship between
bound moisture and production. Here one can
see that corn can either be too wet or too dry,
either of which will reduce the production rate.
Optimum bound moisture content is in the 10
to 12% range. Milo performs in much the same
manner as corn. Therefore, this ingredient must
be handled similarly.
If feed distribution is controlled properly, with
material spread across the entire die, production
capacity of the pellet mill is increased. Consecu-
tive runs of approximately 12 tons each were
produced on a 125 HP pellet mill. These formula-
tions were turkey ﬁnisher with approximately 80%
milo. The aim of the production was to produce
quality ﬁrst and rate second. The ﬁnes were
screened and check weighed to produce results
shown.
Plate #l9 shows the effects on production rate.
Plate #20 shows the effects on pellet tough-
ness.
Plate #21 shows the effects on ﬁnes in the sys-
tem.
26

28.
Plate 19
Moisture vs Production
Plate 20
Moisture vs Durability
Plate 21
Moisture vs Fines
Category V - High Urea Feeds
These formulations contain 6 to 30% urea and/
or urea in combination with molasses. The
key factor to remember in pelleting these feeds
is a severe restriction in the use of steam. The
limitation on this steam addition occurs in the
ﬁnal pellet bin. Any factor that tends to dilute the
urea prill and make it go into solution will create
problems. Urea is soluble in water, so the water
available in molasses alone can create prob-
lems. Also, when urea is heated it reacts to give
off more moisture, accelerating the problem. As
the pellet begins to cool, water with the urea in
solution begins to migrate toward the outside of
the pellet. When it reaches the outside, the wa-
ter evaporates and is drawn off in the cooling air
stream, leaving a concentration of urea on the
surface of the pellet. Urea has an afﬁnity for
water, and therefore can attract moisture as it
stands in a bin. This causes the pellets to be-
come sticky and glue together in the bin.
Binders
In some instances there may be very limited per-
centages of natural binders in the product being
pelleted. Added binders may prove advanta-
geous in this situation. Historically, there has
been a reluctance to add binders, particularly
27

29.
when these binders do not add to the feed value
of the ration. Many binders are now designed to
contribute to feed value and thus are ﬁnancially
justiﬁed.
Much data has been gathered on binder efﬁcien-
cies, some of it conﬂicting in nature and content.
A careful evaluation of characteristics should be
completed before including a binder in the for-
mulation. Speciﬁcally, we must evaluate binders
at the conditioning temperatures and production
rates used on the formulation. Beyond this point,
binders become a matter of personal preference.
G. MAINTENANCE
This paper has thus far discussed equipment
selection, formulation and operation. The fourth
major factor in a successful pelleting opera-
tion is a good maintenance program. There
are two basic underlying facts in a successful
maintenance program.
1. A fast, ﬂexible program is recommended with
strong emphasis on preventative maintenance.
Experience shows great cost advantages with
preventative maintenance to catch minor prob-
lems as they occur. As problem areas are permit-
ted to grow, there is a great acceleration in the
money and time required to correct the deﬁcien-
cy.
2. Single point responsibility. One person
should be assigned responsibility for mainte-
nance of a pellet mill. This clearly establishes
the lines of responsibility and eliminates excuses
for poor performance. Experience indicates that
single point responsibility involves personnel
more fully in the overall performance of the pellet
mill. Pellet mill operation then improves.
Feeder and Conditioner Maintenance
A few basic areas on this unit require mainte-
nance. First, careful attention must be paid to
wear on the conditioner agitator. The tips of
the paddles will eventually wear away and reduce
effectiveness of blending steam and molasses
into the product. This should be reviewed regu-
larly. Also check for bent paddles due to foreign
material. Excessive paddle clearances produce
variations in material conditioning and rate, which
induces erratic pellet mill performance.
Proper paddle adjustment is required, loading
the conditioner 1/2 to 3/4” full to fully utilize the
conditioner volume, and thus get the required
retention time.
Observe proper lubrication schedules to get
maximum life from bearings and seals. Greases
should be selected for proper load-bearing char-
acteristics, with careful attention given to tem-
peratures at which the equipment operates. They
obviously should not be water soluble to minimize
breakdown from steam.
Bearing temperatures in the conditioner can
exceed 200° F. , and greases should be speci-
ﬁed accordingly. Excellent programs offered by all
major lubricant manufacturers. One should take
full advantage of these programs, to get the lubri-
cants most appropriate for the application
The matter of grease seal maintenance is
often overlooked. This speciﬁcally relates to
lip-type sealing elements. Many times, the seals
themselves are replaced but no attention is given
to the surface on which the seal rides. This sur-
face can be abraded away and the seal cannot
function, thus permitting steam and dirt in the
bearings.
It is recommended the conditioner cleanout
be scheduled at the end of each shift to prevent
excessive build up on the walls. Many companies
do this to minimize wear on the agitator and at
the same time provide a smooth, even ﬂow of
feed through the conditioning chamber, guaran-
teeing a better pellet mill operation.
Die Maintenance
Feed Distribution - Proper feed distribution is a
major factor in the productivity and life of pellet
mill components. There must be an even ﬂow of
feed to each individual roll, and this feed must
spread in an even mat across the face of the die
ahead of each roll. Therefore, careful attention
must be given to adjustment of the plows di-
recting the feed to the die. Feed plows are set by
the pellet mill manufacturer at an angle to meet
average conditions. It is not physically possible
28

30.
to set each feed plow to meet the variations of an
individual installation. Thus, feed plow adjust-
ment becomes an operator responsibility.
The ﬂow characteristics of the materials in differ-
ent formulations vary. These ﬂow characteristics
relate speciﬁcally to bulk density, ﬁber content,
etc. High bulk density ingredients such as
ground corn have a tendency to ﬂow quickly
to the back of the die. Higher ﬁber ingredi-
ents such as alfalfa do not ﬂow easily, so they
must be forced to the correct position on the
die. It is impractical to change feed plow position
with each formulation. The feed plow must be
adjusted to get even die wear over an extended
period of time.
Proper feed distribution is imperative from the
minute the die is installed, so all the die begins to
work initially.
There are two ways to evaluate feed distribution
on the die. The ﬁrst method is to check the
wear on the face of the die after it has oper-
ated 24 hours. To do this, one simply cleans
the face of the die, gets a strong light and then
closely observes the wear on the entry into the
individual holes in the pellet mill die. Areas with
the highest feed rates will show more wear.
The second method is to observe the pellet
mill in operation using a strobe light. When
properly set, the strobe freezes the pellets as
they exit from the die and one can clearly see
variations in feed ﬂow if feed distribution is incor-
rect. The feed plow should be adjusted for proper
distribution and should then be maintained in that
condition. Make maintenance notes of correct
feed plow position so it can be duplicated in the
future.
Many people have seen dies that are worn 1/4”
deeper on one side of the die than the other. Not
only does this reduce the usable life of the die,
it decreases the pelleting rate through the mill.
Such wear is due to improper feed distribution.
The normal course of events is as follows: A die
begins to wear on the back and eventually
that portion of the die moves away from the
roll to the point where slippage occurs. When
the operator attempts to set the roll, he be-
gins peening the high or non-wearing portion
of the die, which then accelerates the uneven
wear characteristics. The die will eventually
have to be removed and reworked, although
some people attempt to correct this by revers-
ing the die. This is a Band-Aid effort: it does
not deal with the cause of the problem.
Roll Adjustment
As discussed in the ﬁrst part of this paper, proper
roll adjustment is critical to the operation of the
pellet mill. It is controlled contact with the die
that actually causes the roll to turn. First and
foremost, the roll must be round and rotate
without eccentricity on its bearings. Some
vendors cannot guarantee this. Check before
installing a new roller assembly.
Unfortunately, the standard method of determin-
ing when a roll needs reset is simply to wait until
the pellet mill begins to slip and plug. Only then,
when the pellet mill cannot operate, are rolls re-
set. Assume one is going to wear a die 1/4” deep.
If the die lasts 25 days, it means that the die is
wearing away from the roll at a rate of .010” per
day. Wear rate is therefore a key factor in deter-
mining how often one should reset the rolls. With
a very abrasive pelleting operation, the roll
should be set at least once a day. Many suc-
cessfully pelleting installations only set the rolls
every few days, again dependent on formulation.
Individual experience will dictate the best sched-
ule.
Tramp Metal
Tramp metal is a signiﬁcant factor in die life.
Whenever tramp metal ﬁlls a hole, feed ceas-
es to ﬂow through the hole. Besides reducing
productivity through the die, that particular hole
does not wear and begins to stand up above the
face of the die, looking like a little volcano at this
point. When these projections stick up above
the face of the die, it is impossible to set the rolls
properly. To avoid this situation, maintenance pro-
cedures should be established to punch out the
tramp metal in the die.
Proper magnetic protection, both before the
29

31.
pelleting system and within the pellet mill
itself, is also critical to controlling the metal
problem. Speciﬁc maintenance schedules
should be set to clean the magnets. Proper
magnetic protection also minimizes die break-
age due to shock loading.
Whenever a pellet mill is shut down for an ex-
tended period of time, the die should be ﬂushed
with an oily mixture to condition and protect the
die. This procedure prevents corrosion in the die
due to moisture and acidic ingredients. It also
makes the die start easily when one goes back
into production. Example: Shut down and let a
formulation such as a starter ration with high
sugar content remain in the die holes. The sugars
in the feed will rapidly heat due to the remaining
temperature in the die and can eventually burn to
the face of the die. It will be practically impossible
to start the die again. This is where one peens
the die for that supposedly unknown reason. The
operator simply starts tightening the roll to make
the die pellet - and the roll ruins the die.
Die Removal -- When is a die worn out?
There are many reasons a die is removed from
the pellet mill. The criteria for removal vary with
the installation. A die can be removed for varia-
tion in product, ingredients, sales approach,
maintenance parameters, competition, manage-
ment philosophy, etc. Thus, a die that is worn
out for one person may only be well broken in for
another. The following listing shows the many
reasons for die removal.
a. The die is worn so deeply that the rolls cannot
touch the die, and the pellet mill will not accept
the feed, take steam, etc.
b. The durability index of the pellets produced
has dropped to the point where pellet quality is
no longer acceptable to the customer or sales
department (one must be sure it is the die that is
the problem rather than a shift in ingredient qual-
ity, moisture, formulation, etc.)
c. The die is creating too many ﬁnes. Although
ﬁnes are removed by the sifter, there can be such
a high recycle rate that the system consumes ex-
cessive production time and power. (We recom-
mend you check percentage recycle in the ﬁnes
return system on a regular basis).
d. There has been a shift in the ingredient market
causing reformulation of such a signiﬁcant na-
ture that the existing die is either too thick or too
thin for the formula.
e. The carbon steel die has become so corroded
that its rough surface causes production rate to
drop to an unacceptable level.
f. The die has become ﬁlled with tramp metal
to the point where production is reduced. This
category would also include accidental mixing
problems that cause high percentages of miner-
als to plug a die or burn it shut.
NOTE: A die is worn as material ﬂows through
the holes in it. If a hole in the die is plugged
with foreign materials, then obviously feed
cannot ﬂow through, and it does not wear
down like the rest of the die. Over an extend-
ed period of time, this plugged hole would
begin to stand out above the face of the die. If
allowed to continue, this can stand up so high
that the rolls cannot be set properly, i.e., close
to the die face, and the pellet mill will begin to
perform erratically. Therefore it is important to
remove the tramp objects as soon as the die
hole is plugged. Such conditions also cause
stress in the die from roll contact; this stress
can cause die breakage.
g. Once a die begins to wear below the grooves
cut in the face, it begins to put a higher loading
on the ends of the roller shells, accelerating roller
wear. This is particularly signiﬁcant on hard face
roller shells where the shells could be used to
wear out a second die. By attempting to get a
little more wear out of the die, one can de-
stroy two or three roller shells and possibly
the bearings, which can be more valuable
than the remaining life in the die.
h. The die has been peened too badly. The die
should be removed, reworked and then re-in-
stalled on the pellet mill.
30

32.
I. The hole diameter of the die has grown to
the point where pellet diameter is too large for the
customer to accept. (Note -- this is a much more
prevalent situation in a carbon steel die.)
j. If the die ﬁt area has deteriorated to the point
where the die is much too loose, it can cause
accelerated wear on the wear ring, clamp ring,
die housing, etc. which in turn can result in high
maintenance costs.
k. The die is cracked due to tramp metal, mis-
treatment, poor maintenance procedures, etc.
l. There is grossly uneven die wear across the
die face due to poor feed distribution, worn feed
plows, distributors out of adjustment, etc. This
uneven wear reduces production rate and pel-
let quality. At best, the die should be removed,
the face trued up (ground) and then re-installed.
At worst, the die should be discarded if the life
remaining does not justify rework.
Die Fits
Proper die ﬁts must be maintained at all times,
because the die must have support on both
sides to withstand the forces generated in the
pelleting operation. 80% of die breakage prob-
lems exist as a direct result of improper die
ﬁts. In calculating average die life, one must
consider broken dies.
Roller Assemblies
The key factor in roller assembly maintenance
is proper bearing setting so that the roll runs
true and maintains a proper relationship with
the die. This is not possible if the roll shell is not
round. Other features are proper lubrication
and proper seal maintenance. The grease in
the roller assembly goes well beyond lubricating
the bearings themselves; greasing also serves to
purge foreign materials from the bearing assem-
bly. Experience indicates greases with extreme
pressure additives provide distinct advantages on
most applications. It should also be noted if the
roll face wears unevenly, it can become impos-
sible to adjust the roll properly to the die face.
Seals
It is important to maintain the seal where the
mash leaves the stationary feed spout and
enters the rotating pelleting chamber. When-
ever excessive clearance develops in this area,
it permits the mash to bypass the pelleting cham-
ber and drop into the ﬁnished product. This can
create difﬁculties in cooling or sifting equipment
and increase the potential for ﬁnes in the ﬁnal
pellets.
Boiler Maintenance
It is important to have dry steam free of conden-
sate coming to the conditioning chamber in a
pellet mill. Proper boiler maintenance helps guar-
antee this condition. In particular, it is mandatory
that boiler chemistry be properly maintained.
If not, surging and heaving will occur at the water
surface line, creating wet steam conditions as
excessive water is carried into the steam lines.
This can then be carried through to the pellet mill,
causing the mill to plug.
With energy costs rising, it is imperative that the
boiler be adjusted for maximum efﬁciency at all
times. Out of spec chemistry within the boiler can
affect heat transfer rates. If there is a scale build-
up on the heat transfer surfaces, efﬁciency will
drop. In most instances, boiler maintenance is
contracted to guarantee proper feed water condi-
tioning.
Finally, all steam traps and water removal
piping systems should be maintained in top
condition to minimize condensation to the
conditioning chamber.
31

33.
PELLETING - AFTER THE DIE
The fundamental factors concerning the cooling,
crumbling and grading of pellets are as signiﬁcant
as the fundamentals of pellet formation.
A pellet is in its most fragile state as it leaves the
die. It has been formed but is a soft plastic, easily
deformable product at this time. Every effort must
be made to handle this product as gently as pos-
sible until it is cooled, dried and hardened. From
a system standpoint, the pellet should drop
directly from the pellet mill into the cooler,
since any type of mechanical handling will gen-
erate ﬁnes. If for some reason a layout requires
handling between the pellet mill and the cooler,
potential breakage should be considered. For
instance, a belt type conveyor has proven to be
one of the best mechanisms used to convey hot
pellets to a remote cooler.
A. Cooling Equipment - Theory and Operation
There are three basic types of coolers used in the
feed industry today: the horizontal cooler, the
vertical cooler, and counterﬂow coolers. There
are basic advantages to each type of cooler but
the same theory of operation applies to both.
1. How Pellets Are Cooled
The pellet cooler performs two functions on
the pellets. As it enters the cooler, both moisture
and heat are removed at the same time and in a
well-established order. The lack of either heat or
moisture will affect the performance of the cooler.
The basic parameters existing in the conditioning
process also exist in the pellet cooler. Therefore,
if we lower the temperature of the pellet 20°F, we
can expect a 1% reduction in pellet moisture. Pel-
let coolers are able to remove most of the heat
and moisture added from the stream conditioning
process and the heat added from the main motor.
Step by step, here is what happens:
a) Steam condenses on the mash in the con-
ditioning chamber, causing the moisture level of
the mash to increase on an average, 3 to 5%. In
condensing steam, large quantities of heat are
gained. This mash is then pelleted and more
heat is added through friction and mechanical
working. Pellets are then discharged with the out-
let temperature averaging somewhere between
140 and 200°F. At this point, the pellets require
cooling and drying to get a durable product.
b) As it leaves the pellet mill, the pellet has a
relatively ﬁbrous structure, allowing moisture
to migrate by capillary action. This is the same
mechanism present when moisture is picked up
with a paper towel or ink is being blotted.
c) The pellet cooler is designed to bring ambient
air in contact with the outer surface of the pellets.
This air, assuming it is not 100% saturated,
will pick up moisture from the pellet surface,
where it is most readily available. The moisture
evaporates, causing cooling as the moisture
moves into the air.
d) Heat picked up by the air increases air
temperature, which in turn increases its ca-
pability to pick up water. Conversely, this
heat is required to avoid condensation in the
air system due to the added moisture. For
example, if the air in the cooler was 70°F with a
relative humidity of 85% and this air was heated
by passing through a bed of pellets to 120°F, its
moisture carrying capacity would be 5 times more
than in its original state. However, there has been
a pick-up of moisture in the cooler, and there is a
delicate heat-moisture balance.
e) The pellet is left in an unbalanced condi-
tion when surface moisture is picked up by the
cooling air. More moisture is concentrated in the
center of the pellet than on the outside. Because
of this unbalanced condition, the pellet behaves
like a wick, causing moisture to migrate to the
pellet surface along with heat. This moisture is
then available for pick-up by the cooling air.
f) This process continues until most of the mois-
ture added in the conditioning stage is removed
along with the heat. Moisture remaining in a
pellet is usually equal to or slightly more than
the bound moisture of the ingredients as they
come to the conditioning chamber. This bound
moisture will not be removed in an ambient air
cooler under normal circumstances. The excep-
tion exists when large volumes of extremely
32

34.
dry air enter a pellet cooler and cause an actual
loss of moisture or "shrink". Special ambient
conditions must exist for shrink to be a problem.
Conversely there can be times when water has
been added to the mash before the condition-
ing chamber and not enough heat is available to
drive off this moisture. Under these conditions,
you will have higher ﬁnal pellet moistures.
g) This is an ambient air-type cooling process, so
the pellets will always be discharge at temper-
atures higher (10 to 15°F) than the tempera-
ture of the air entering the cooler. This means
if the air enters the cooler at 60°F, the pellets will
be discharged between 70 and 75°F.
2. Pellet Temperature
It is a well-known fact that the hotter the pellets
going into the cooler, the more efﬁcient the
drying process will be. High temperature pellets
do three things:
a) They heat the air, giving it more capacity to
take up moisture.
b) The heat in the pellet provides energy to
move the moisture more rapidly from the center
to the surface where it can be removed.
c) Moisture leaves a warm surface faster than
a cold surface because the temperature of the
moisture itself is higher. Remember how much
faster moisture leaves a dinner plate rinsed in ex-
tremely hot water compared to one that has been
rinsed in cold water. For this reason it is best to
put the pellets in the cooler as quickly as possible
in order to take full advantage of the heat con-
tained in the pellets.
3. Cooler Selection
The selection of a cooler for a given job involves
the following steps:
a) Determination of the type of cooler : hori-
zontal or vertical--Either a vertical or horizontal
cooler will do an excellent job of cooling pellets.
Plant layout and product mix will determine which
cooler to use. Obviously where the ﬂoor space
is limited, the vertical cooler will be most
appropriate. Where height is limited, as in a
basement location, the horizontal cooler will
be used. The features of both types of cooler
are listed below:
Vertical Cooler - The vertical cooler is normally
best for a small diameter pellet if the height
is available for installation. First, the design is
simple, minimizing maintenance cost and energy
costs. As is seen in Plate #22, the pellets are
directed into the top hopper of the cooler where
they are diverted by the stream splitter into col-
umns approximately 9” wide. Pellets ﬁll these two
columns until the control vane at the top actuates
the discharge control mechanism. As the pellets
ﬂow through, they are exposed to high velocity
air which cools and dries them. The air is drawn
through the pellets via a fan connected to the
center section. The pellet discharge from a verti-
cal cooler has a very smooth, constant ﬂow rate,
making it ideal for feeding crumbles rolls.
Plate 22
Verticle Cooler: Input regulates Output
Note: Sketch 1, that while the cooler is ﬁlling, control gates
hold pellets in the unit. As the Cooler ﬁlls, Sketch 2, pel-
lets build up in the column and depress the control vane
to raise the feed control gates through direct mechanical
linkage.
Flow out of the Coolaire™ cooler is regulated by the
amount of feed coming from the pellet mill...assuring uni-
form cooling and drying for a superior ﬁnished product!
33

35.
Horizontal Cooler - The horizontal cooler is a
moving apron-type cooler. It differs from the verti-
cal cooler in that pellets remain stationary and
move through the cooler on the apron; whereas
on the vertical cooler pellets are agitated as they
move down the column under gravity. The other
obvious difference, of course, is the fact that the
direction of ﬂow on a horizontal cooler is in a hori-
zontal plane instead of vertical.
The horizontal cooler is made in two basic types:
(a) The single pass unit
(b) The double pass unit
The single pass unit has one moving apron or
belt and the pellets discharge at the end opposite
the inlet. On the double pass cooler there are two
moving aprons. The pellets move with the top
apron, drop down onto the bottom apron and are
discharged at the inlet end.
Plate 23
Horizontal Cooler
In the horizontal cooler, cooling air is introduced
at the bottom, ﬂowing vertically upward through
the moving beds of pellets to where it is drawn
through a hood into the duct work and to the fan.
A basic horizontal cooler is shown in Plate #24.
The pellets themselves are fed onto the moving
apron in a variety of manners. In some instances,
they are simply choke fed with the apron draw-
ing pellets from an inlet hopper across the entire
width of the cooler. Sometimes the manufacturer
supplies an oscillating feed spout which moves in
a semicircular motion across the apron, deposit-
ing the pellets across the width.
As the ﬁnes in the horizontal cooler sift down
through the pans they are eventually deposited
on the bottom of the cooler. Here rubber ﬁnes
wipers scrape the product to one end, lift it and
drop it into a trough attached to the scraper. The
ﬁnes are then carried along with the pellets and
discharged at the end of the cooler.
34

36.
This design cooler is best for a very fragile
pellet or for cubes. In cube production, a lon-
ger retention time is required in order to properly
cool and dry the cubes. The space required for
the cooler is much larger. There are also fewer
mechanical problems handling the cubes in a
horizontal cooler than in the vertical design. The
large, long cubes, in many instances, have a
tendency to hang up in the discharge mecha-
nisms of vertical coolers.
Due to the conﬁguration of the horizontal
cooler, there is less tendency to pull ﬁnes into
the air stream. There is a plenum chamber effect
in the hood which reduces the air velocity and
permits the large ﬁnes particles to drop back onto
the pellet bed.
The horizontal cooler is more involved from
a mechanical standpoint. More moving parts
mean higher maintenance costs. The tray-type
horizontal cooler has a surging discharge charac-
teristic that creates problems in feeding crumbles
rolls. Therefore, proper attention must be given
to spouting, speed adjustment and bed depth to
achieve the best crumbling performance.
b) Determination of retention time -- The time
it takes for moisture in the center of the pellet to
move to the surface and be evaporated is known
as the retention time. The retention time required
to cool a pellet depends upon its size and compo-
sition. The retention time in a cooler is calculated
by comparing its volume to the production rate of
the pellet mill and to the pellet bulk density. This
is expressed by the following equation:
T = (V)(D)60
R
where:
T = Retention time in minutes
Y = Volume of cooler in cubic feet
D = Bulk density of the pellets (Lbs./cu.ft.)
R = The production rate in lbs./hr.
Plate #24 gives recommended times for vari-
ous pellet sizes, in both horizontal and vertical
coolers. Retention times are based on test data
gathered from the ﬁeld. They should be varied for
unusual operating condition.
c) Determination of cooler size based on
retention time -- All formulations should be re-
viewed for cooling requirements before the cooler
is selected. For instance, if you are making both
small and large pellets, it may be the cubes that
determine the ﬁnal cooler size rather than small
pellet production.
These retention times are for general formula
feeds containing not more than 5 to 10%
liquid feed ingredients. Generally, a six min-
ute retention time is preferred for small diameter
pellets. There are times when you will reduce
retention times, down to 5 minutes. This is right
on the borderline and will require extremely good
conditioning of the mash to dry the pellets in time.
In addition, ﬁnes must be held to a minimum. Un-
der no circumstances should retention time ever
be less than 6 minutes. There are exceptions
to these times, for instances, 1/4" alfalfa pellets
should have 8 minutes retention time. On pel-
lets containing more than 10% molasses, one
should increase the retention time, at least
20%. Use of these retention times assumes
that adequate air volumes are being used.
d) Determination of air volume required -- The
air volume required relates directly to the
production rate being processed through the
cooler. It does not relate to cooler size or pel-
let mill size.
Air required for adequate cooling also depends
upon the pellet size. Plate #25 provides you
with data required to select the amount of air for
proper cooling and system functioning.
Problems with condensate in air systems
are directly related to the volume of air used
to cool the pellet. There are large variations in
the relative humidity of the air coming to a pellet
cooler. Therefore, allowances are made to com-
pensate for these variations.
The following example shows the method of
determining the air rate required: Assume a pel-
let mill is capable of producing a maximum of 20
TPH of 10/64 diameter pellets.
35

37.
Minimum Retention Time For Most
Formula Feeds
Pellet Size Retention Time
10/64” to 12/64” 5-6 minutes
1/4” 6-8 minutes
3/8” 7-8 minutes
1/2” 8-10 minutes
3/4” 12 minutes
4/8” 15 minutes
1/4” Alfalfa Pellets 8 minutes
Plate 24: Recommended Cooler Retention Time.
SOME MATERIALS MAY REQUIRE LONGER
RETENTION. FEEDS CONTAINING MORE
THAN 10 PERCENT MOLASSES REQUIRE 20
PERCENT MORE TIME THAN SHOWN ABOVE.
Plate 25: Cooling Air Requirements
One simply multiplies the 800 CFM of air required
per ton of pellets by the 20 RPH production rate
and gets an air requirement of 16,000 CFM.
The foregoing recommendations should be tem-
pered with experience on particular feeds and lo-
cal weather conditions. For example, if the rela-
tive humidity in an area generally is between
85 and 100%, it might be wise to increase the
air volume. On the other hand, an extremely dry
climate means less air can be used. For general
conditions, however, the above chart should be
adequate.
Having determined the air required, one must
now go back and check the cooler to be certain
this volume of air is within the velocity limits of
the pellet bed. Check to be sure you are not
exceeding maximum velocities on any design
cooler.
Experience indicates that on vertical type cool-
ers, we should limit air velocities to 350 ft./min.
On horizontal coolers, it can be raised to 580
ft./min. Note that these velocities are velocities
through the open area either in the screen of the
vertical cooler or through the perforations of the
trays on the horizontal cooler. They do not relate
to the total cross sectional area of the cooler.
A horizontal cooler has fewer problems with
ﬁnes pick-up than a vertical unit, because
the ﬁne particles actually have to be lifted
from the pellet bed by the air before becom-
ing entrained in the air system. However, in
the vertical cooler, any particle passing through
the screen can (and must) be induced into the air
system. Here gravity is assisting particle move-
ment, as seen in the horizontal cooler. Properly
designed horizontal cooler hoods do not permit
air velocities high enough to pick up any but the
smaller dust particles.
In addition, there is a potential for holding the
ﬁnes and pellets against the interscreens on a
vertical cooler unit. The prescribed velocities
avoid this situation and the resultant choking
down of the air system.
This cooling concept utilizes a true counterﬂow
principle where air ﬂow is 180° to the pellet ﬂow
though the cooler. This causes the coolest air
to ﬂow over the coolest pellet and, conversely,
the warmest air over the warmest pellets. This
provides maximum cooling and the most cost ef-
fective use of the cooling air. Plate #26 illustrates
how the counterﬂow functions.
The unit is essentially a rectangular box; the bot-
tom is an oscillating grate that both controls pellet
discharge and permits entry of cooling air. The
cooler top utilizes a rotary valve type airlock for
both pellet entry and an air seal. The air is pulled
through the pellets and out through an exhaust
Minimum Retention Time For Most
Formula Feeds
Pellet Size Retention Time
10/64” to 12/64” 5-6 minutes
1/4” 6-8 minutes
3/8” 7-8 minutes
1/2” 8-10 minutes
36

38.
hood in the top section. Dependent on cooler size
and pellet type -- various inlet distributors are
available to evenly distribute the pellets across
the entire cooling area.
The oscillating discharge grate operates in an on/
off mode, actuated by high and low level indica-
tors mounted on the side of the cooler. These
level controls are adjustable to control pellet
retention times in the cooling area.
Unique features of a counterﬂow cooler are:
1. Lowest initial cost
2. Maximum cooling efﬁciency per air volume
providing energy savings from minimum fan size
and IP (Utilizes air ﬂow rates between 500 and
670 CFM per TPH of pellets)
3. Low maintenance
a. Few moving parts
b. No trays
c. No screens
Plate 26: Principle of the Counterﬂow Cooler
4. Compact design provides maximum cooling
capacity for a given cooling area and height
IMPORTANT: Retention time and air volume are
independent considerations when sizing the cool-
er. A given cooler size does not necessarily
require a certain amount of air. Note also, that
we must size for the ﬂow of product through
the cooler, not for ﬁnished product rate. If the
system is recycling excessive ﬁnes, the extra
production rate through the cooler can cause
increased ﬁnal pellet temperatures. The normal
retention times and air ﬂow rates handle average
ﬁnes conditions, but if one notices warmer pellets
at the cooler discharge, the overall production
rate through the pellet mill should be checked.
The ﬁnes normally in the hot pellets coming to
the cooler should be distributed evenly through
the pellets. If all ﬁnes are concentrated in one
particular area, they will completely ﬁll the voids
between the pellets and block off the ﬂow of cool-
ing air.
Pay careful attention to the spouting that brings
pellets to the cooler inlet. Angular inlets will cause
ﬁnes concentration. Therefore, we recommend
the pellets be spouted vertically into the
center of the cooler inlet hopper. Experience
has shown that this permits the ﬁnes to distribute
themselves evenly, avoiding hot spots in a pellet
cooler discharge.
B. Fans
Air system fans should be located in the
negative pressure side of the collector for
maximum efﬁciency as shown in Plate #27.
Experience has shown that dust particles coming
from a pellet cooler are normally quite large and
thus easy to collect. When you position the fan
after the collector, you avoid the impingement of
dust particles on the fan impeller with the resul-
tant breakdown into ﬁner dust.
37

39.
The fan itself must be sized to handle all sys-
tem pressure losses at the rate of ﬂow selected.
These losses can be broken down into the follow-
ing categories:
1. Loss through the cooler and pellets.
2. Transition and duct work losses.
3. Collector losses.
Normally, loss through a cooler and its bed of
pellets is between 1 1/2 and 2" of water. Both
the duct and collector losses will vary with the
system design and the type of collector selected.
Radial wheel type fans are normally selected
for pellet cooling applications because of their
dependability. The inclined blade type of impel-
ler is normally used on fans located after the dust
collector. At this point there are no ﬁnes to cause
abrasion problems with the impeller and the in-
creased fan efﬁciency keeps power demands to a
minimum.
Air system selection - the fan and collector size
are dependent on the cooler size and the air
required. The duct work should be sized to mini-
mize pressure losses yet keep the ﬁnes in sus-
pension. The collector should always be located
as close to the cooler as possible.
Wherever possible, the fan should be located
where it can be periodically checked and
there is access for maintenance personnel
when problems arise.
C. Dust Collector Selection
Cyclone type dust collectors are normally used
because they have acceptable efﬁciencies with
minimum cost and maintenance. The higher
efﬁciency collectors used today do meet the
tightening restrictions imposed by various en-
vironmental protection agencies.
People talk of using ﬁlters on the air coming from
the pellet cooling and drying process. To date,
there is no known installation where ﬁlters have
been successfully used to clean air from a pellet
cooling system. The high humidity of the exhaust
air is the major problem. Those who have tried
this approach quickly ﬁnd that sometimes the
exhaust air reaches a saturated (100% relative
humidity) condition and moisture condenses on
the ﬁlter cloth. Dust then quickly accumulates on
the wet surfaces and it cannot be cleaned. Thus,
it quickly builds up to the point of complete stop-
page.
There have been attempts to heat the air coming
to the ﬁlters to keep it above the saturated condi-
tion, but costs are generally prohibitive--particu-
larly in view of the rising costs of energy.
The efﬁciency of the collector depends on many
things. The more important factors are:
*Design
*Particle Size
*Dust Loading
*System Operation
While a detailed discussion of collector perfor-
mances is beyond the scope of this paper, there
are certain factors that should be presented.
Plate 27: Arrangement - Dust Collector & Fan
38

40.
1. Design
Small diameter cyclone collectors are more
efﬁcient than larger ones because the cen-
trifugal (separating) force for given tangential
(inlet) velocities varies inversely as the radius
of the cyclone. There are practical limits to inlet
air velocities based on the static pressure drop.
It simply takes too much horsepower to further
increase pressure drop in the large collectors, so
the only practical design direction is to reduce the
collector size. There is an additional advantage
in a smaller collector size. It permits the collec-
tors to be more readily installed in a protective
atmosphere to control condensation problems.
It appears that the 54" diameter collector is a
good size limit for best collector efﬁciency. Larger
collectors can be used but will operate at slightly
reduced efﬁciencies. The 54" diameter collector is
adequate for air volumes to 10,000 CFM. Speciﬁ-
cally desirable features of efﬁcient collectors are
as follows:
An involute shaped inlet for minimum turbulence
and reduced potential for by-pass or reintrain-
ment. The collector should be long for proper
vortex length. There should be a cleanout door
provided, large enough for a person to enter if
it is necessary to clean the walls. It is also ad-
visable to have a small door located above the
rotary valve to check the condition of the valve,
assist in clean out, etc.
2. Particle Size
Particle size is the single most important fac-
tor in dust collector efﬁciency. This is a vari-
able in any pelleting operation due to changes in
ingredients, types of grind, moisture pellet quality
etc. The larger the dust particle, the easier it is to
remove.
3. Dust Loading
The quality of the pellet produced has a
signiﬁcant effect on the amount of dust enter-
ing the cooling air system. Normal cooler and
collecting systems are designed to handle pellets
that do not exceed 10% ﬁnes. While collector
efﬁciencies may remain essentially the same for
an excessive ﬁnes condition, the total collector
efﬂuent may exceed required limits of the govern-
ing code.
4. System Operation
Any ambient air cooler-dryer has the inherent
potential of operating with an exhaust air sys-
tem very near saturation. In many measured
instances, it has been found that the median
between the cooler exhaust air, wet bulb and dry
bulb temperatures is less than 10°F or approxi-
mately 80% relative humidity. One can readily
see the great potential existing for condensation.
Therefore, the installation must be designed
to minimize cooling effects. Condensation in
an air system is bad because the resultant mois-
ture impinges on the duct work and the collector
internals. These wet surfaces immediately collect
dust and a rough hard scale begins to form. In
extremely cold climates, this moisture and dust
combination may freeze on the inside of the duct
work, etc. quickly building to an intolerable level.
This build-up can accumulate to the point where
it will completely choke the system and/or falls off
inside the collector in large chunks blocking the
toe of the collector. When this happens, all the
dust will be exhausted to the atmosphere. The
collector, of course, must then be cleaned before
operation can continue.
The connecting 75 to 100 ft. of duct work going
up through a normally unheated bin structure
can cause enough cooling for condensation. The
most effective means of avoiding this is to install
the collector inside the building, preferably in a
small enclosed area that can be heated if neces-
sary. The duct work should be kept as short as
possible. If the enclosed area is properly de-
signed, normal heat losses from the collector and
piping may be sufﬁcient to keep the space warm
enough to prevent condensation. In extremely
cold temperatures, unit heaters may be neces-
sary to maintain proper ambient conditions partic-
ularly during start-up when the piping and collec-
tor are cold. Even if excessive build-ups do not
occur, the collector efﬁciency is impaired if there
is any build-up on the inside causing a rough
surface. Rough internal surfaces on a collector
create turbulence which does reduce efﬁciency.
Any variation in the system that increases static
pressure loss will reduce air ﬂow and increase
condensation problems. Test work indicates
that saturated air will exist if the air ﬂow rate
is too low. Proper use of the air requirements
39

41.
keep the air-to-product ratio at an acceptable
level.
An excessive amount of ﬁnes in the coolers is
the most common cause of reduced air ﬂow,
again demonstrating that good pellet quality is a
major factor as any cooling and dust collecting
system. The ﬁner particles tend to cling to the
screen of the vertical cooler, in particular, rather
than ﬂowing down with the pellets. This build-up
can continue to a point where air ﬂow is partially
to completely blocked. Excessive air ﬂow in re-
lation to screen area creates the same effect.
Air resistance can quickly build up as the screens
begin to close off.
D. Duct Work
As indicated above, duct work should be kept
as short as possible to avoid condensation
problems and reduce losses in the system. Fan
power demands go up in direct relation to in-
creases in static pressure, thereby increasing op-
erating costs. It is necessary to keep air velocities
above minimum levels to prevent ﬁnes from set-
tling out in the duct work. As a general rule, air
velocities should be held between 4500 and
5000 ft/min. Proper duct work design speciﬁes
that elbows should have generous radius to keep
losses and abrasion to a minimum. All elbows
should be smoothly contoured for minimum pres-
sure losses, and all duct work transitions should
be gradual.
E. Installation of Cooling Equipment
Cooling equipment poses no particular problems
in installation. The units can be shipped assem-
bled or knocked down depending upon the manu-
facturer’s design and contractor requirements.
Care should be taken when installing equipment
to allow the space required for maintenance.
Coolers should be located where they can
receive fresh air from the outside. If in-plant
air is used, the pellets will only be cooled to 10 to
15o above the in-plant air temperature. This can
cause problems. For example, an installation in
New York State using in-plant air for coolers had
the coolers receiving approximately 50 to 60o
F air in the winter. The pellets were thus being
cooled to temperatures between 60 and 70o F.
Since every last drop of moisture cannot be re-
moved from the pellets, when they were put into
a boxcar outside, in temperatures between 10 to
20 o F, the warm, slightly moist air surrounding
the pellets rose to the roof of the car. Upon touch-
ing the cold boxcar roof, this air reached its dew
point; and the moisture condensed, then dripped
back onto the pellets, causing spoilage. The
problem was eliminated by cutting louvers in the
wall to allow an adequate supply of outside air
to enter at its reduced temperatures. This gener-
ated further cooling of the pellets and solved the
problem.
There may be certain limitations to extremely
cold air in the northern climates. Instances
have been reported where very cold air drawn
through a cooler froze the outer skin of the
pellets, prohibiting further moisture migra-
tion. In such instances, heating coils to raise
the air above freezing point will help avoid
this problem.
F. Maintenance
A preventative maintenance program should be
established to catch problems in their early stag-
es. We suggest that the following areas be incor-
porated in such a program: The vertical cooler
inner screens should be checked regularly
for wear, since holes permit a constant ﬂow
of pellets into the air stream. Rotary Valve
clearances should be checked regularly to
be sure the valve is producing the proper air
seal at the toe of the collector. Maintenance
access should be provided around the rotary
valve so it receives proper attention. The duct
work should be checked periodically to keep
the system as tight as possible. Air leakage,
particularly just ahead of the fan, can cause
signiﬁcant reductions in air ﬂow through the
cooler and collector. This reduces both cool-
ing and collecting efﬁciency.
Pellet quality should again be mentioned within
the scope of this section. Excessive ﬁnes cause
a large number of air system problems. Varia-
tion in ingredients, particularly, as a result of least
cost formulation, as well as the magnitude of
other well-known problems, can and does affect
the ﬁnes percentage in the pellets going through
40

42.
a cooler. One can readily see the advantage of
day-to-day quality control programs at the plant
level to maintain proper conditioning for optimum
pellet quality.
For all practical purposes on normal feed mill
applications, the collector efﬁciency is dependent
upon dust loading in the cooling air. This can be
a problem because the amount of dust leaving
the collectors is then directly dependent upon the
dust load entering the collector. For example, if
you have ﬁve times as much dust as normal in
the cooling air, the collector will dust ﬁve times
the normal rate. If there is excessive dust load-
ing in the cooling air, it is entirely possible you will
violate the applicable code governing your opera-
tion. As a general statement, high-grain poultry
feeds normally would produce more ﬁnes than
dairy feeds, and we would expect more dust from
this type of operation.
G. Crumblizing Equipment
1. Design & Operation
Crumbles rolls are used primarily in the poultry
industry. They reduce cold pellets into small par-
ticles called crumbles. Young chickens accept
crumbles at an earlier age than pellets and there-
fore are usually fed crumbles during the third and
fourth weeks. Then the chicks are switched to
pellets for the remainder of the feeding period.
Some large growers, however, feed crumbles
throughout the entire feeding period.
Crumbles are usually made from a 3/16 or a
5/32 pellet because these particular sizes have
a high production rate at the pellet , yet are small
enough to crumble easily without making too
many ﬁnes. The objective of crumbling is not
to merely reduce the size of the pellets but
to control the reduction to a speciﬁc particle
size with a minimum of ﬁnes.
The crumblizer is actually a roller mill. Exten-
sive testing has proved that a roller mill is the
most efﬁcient reduction unit for reasonably friable
material. A roller mill cuts the material cleanly be-
tween rolls with very little attrition of material on
material. Therefore, power demand is relatively
low, and the ﬁnes produced are held to a mini-
mum. Low power consumption and percentage of
ﬁnes makes a very economical installation.
For highest efﬁciency, we must keep pellet
diameters small to get the proper relationship
to roll diameter. With a larger diameter pellet,
there is a greater concentration of material in a
given spot causing attrition of material on material
and resulting in higher percentages of ﬁnes and
higher power consumption. Also, if the pellets are
too large in relationship to the roll diameter, the
gripping or feeding efﬁciency is reduced. On a 6”
diameter roll, the pellets should not exceed
3/16 diameter as feed stock. On the 9” diam-
eter roll, the pellets should not exceed 1/4”
diameter.
The gap or setting between the two rolls af-
fects crumbling efﬁciency and the diameter
pellet that the rolls can accept. The capacity
ratings for crumbles rolls are based on a setting
where the gap between rolls is 2/3 the diameter
of the pellet to be crumbled. Efﬁcient operation
of a roller mill is only obtained when there is a
thin curtain of feed passing through the nip
between the rolls. Because of the cracking ac-
tion of the crumbler, it is imperative that the
pellets fed to the crumbler be spread across
the entire width of the rolls--not concentrated
in a small area. Actually the ideal ﬂow of pellets
to a crumbler is a thin fast ﬂowing stream, as we
strive to have the corrugations crack each pel-
let individually. This produces crumbles in range
with minimum ﬁnes. Crushing occurs when choke
feeding a mass of pellets, thus, generating very
little in range product and an excessive amount
of ﬁnes.
2. Roll Design
The main components of a crumbles roll are two
hardened and corrugated cast metal rolls. The
fast roll that acts as a feed roll is cut longitudi-
nally and the slow roll circumferencially. This
is the most common type of corrugation for pellet-
ing feed.
Certain companies, however, are convinced that
longitudinal sawtooth-type corrugations on both
rolls provide the product they want and keep the
ﬁnes to a minimum. Thus, corrugation becomes a
matter of product and preference.
41

43.
These rolls are mounted on anti-friction bear-
ings bolted to a rugged steel frame to guarantee
proper alignment of the rolls, keeping them in
tram (parallel).
The gap setting between the rolls is done by
adjusting screws. Normally, there is an adjust-
ing screw on each side of the frame, requiring an
individual setting to get the rolls parallel. Adjust-
ing screws are usually designed with an internal
spring mechanism so the rolls can part and pass
any hard foreign object that might be in the feed.
There is a bypass valve mounted within the
crumble roll that diverts the pellets to the nip of
the crumbles rolls when crumbles are required
and around them when they are not.
One recent development in crumbles roll
design is an air operated control mechanism
that pneumatically opens or closes the roll.
When producing pellets, the control device is
positioned so that the rolls will open to allow the
pellets to drop completely through the crumbler.
When crumbles are required, the controls are
energized, activating the pneumatic mechanism
to close the rolls to a predetermined setting. This
starts the crumbling operation. The control device
can be remotely located--meaning that the opera-
tor in one part of a plant can, by moving a control
lever, activate the crumbler in another part of the
plant.
There is an additional advantage in the pneu-
matic actuated crumbler--its ability to clear
the crumbles roll whenever it is plugged. The
design of a standard crumbles roll is such that
if we get a handful of hard pellets lying in the
nip between the rolls when it is not running, it is
impossible to start. This situation requires an op-
erator to travel to the crumble roll location, open
the gap between the rolls by turning the adjusting
screws to get a clearance. Then after the rolls
are cleared, he has to turn them to their original
position before beginning operation. With the
pneumatically actuated crumbles roll, the opera-
tor can simply ﬂick the control lever. The rolls will
pneumatically open, letting the surge or plug of
pellets ﬂush through. They will then return to the
original position. This feature greatly reduces
down time.
3. Installation
There are two schools of thought on how to install
crumbles rolls. We will attempt to point out the
advantages and limitations of each. For this dis-
cussion, the systems will be called “Compact”
and “Long”. These two systems give about the
same results, so the difference is in the personal
preferences of those who operate them or in
installation limitations.
First, it must be understood that in order to make
a good crumble and keep ﬁnes to an absolute
minimum, a good quality pellet must be made. It
is more important to make a good pellet when
making crumbles than at any other time. A
quality pellet, in this case, would be one having a
durability of 9.4 or better, (the 9.4 refers to dura-
bility ratings per the Kansas State tumbling box-
type durability test).
The “Compact” system consists of installing the
rolls directly under the discharge of the pellet
cooler. This serves as a uniform feeder the entire
length of the roll, eliminating the need for an addi-
tional feed mechanism. Uniform feeding is neces-
sary to prevent overloading the rolls in any one
area and avoid a crushing action which generates
excessive ﬁnes and reduces production.
With this system, the rolls should always be
maintained and adjusted to keep whole pel-
lets or particles too large to pass through the
top screen of the pellet grader to an absolute
minimum. If not, these overs are returned with
the ﬁnes back to the mash bin over the pellet
mill where they go through the pelleting sys-
tem dry and hard. Here also, they are ground
up on the die and pushed through dry, making
a poor pellet and increasing the abrasion on
the die.
When this compact system is operated as de-
scribed, the percentage of ﬁnes made in the
crumbling operation from quality pellets will rarely
exceed 10%. When one makes very ﬁne crum-
bles, this percentage may reach a maximum of
15%. In certain instances, installations have an
elevator to take the oversized pieces and the
42

44.
whole pellets back to the top of the cooler. Here
they can be processed through the crumbles
roll again rather than returning a large amount
of overs to the pellet mill. The question here is
whether the savings justify the cost of the el-
evator. They would not appear to do so except
where large quantities of coarse crumbles are
made.
Now let us consider the so called "Long" system.
With this system, the crumbles rolls are installed
at some point removed from the cooler. The
discharge of the cooler is conveyed to a receiving
bin mounted over the crumbler. This requires a
special feeder to the crumbles rolls themselves.
There are two common methods of feeding used,
the roll feeder and the shake feeder.
The roll feeder consists of a ﬂuted roll and an
adjustable weir. This feeder is normally driven at
a constant speed from the fast roll on the crum-
bles roll itself. It is necessary to adjust the weir on
this type of feeder each time to handle the pro-
duction rate and get the pellets spread across the
full length of the roll.
The second and preferred method of feeding is
to use a shake feeder to meter the feed evenly
to the crumbles roll. This is more ﬂexible to
variations in pellet production rate and is gener-
ally less of a maintenance problem.
4. Operation
Remember the basic sequence in starting a
crumbles roll. The roll must always be running
before the product is fed to the crumbles roll.
The roll simply does not have the torque to start
under load. Secondly, one should always work
to maintain an even feed rate to a crumble roll,
keeping ﬁnes to a minimum.
Normally, a trial run is required to get the proper
roll adjustment. Normal procedure is to set the
gap and then catch the crumbles coming out from
each end of the rolls. They should be compared
visually or by screen analysis. Again, careful at-
tention must be given to the roll settings at both
ends to keep the rolls parallel. Some manufactur-
ers supply crumbles roll with a single adjusting
mechanism and a geared connection to adjust
both ends of the roll from a single point. This
eliminates the problem keeping the rolls in paral-
lel.
Having sampled the product, decide whether the
crumbles are too coarse or too ﬁne. Should the
crumbles be too ﬁne, increase the gap between
the rolls. If they are too coarse, decrease the gap
between the rolls. Should several size crumbles
be made, gauges should be fabricated and set-
tings recorded in order to duplicate the product.
5. Trouble Shooting
First and foremost, always remember to lock
out the crumbles roll motor before working on
the equipment.
Non Uniform Product - This is generally caused
when the clearance between the rolls is not the
same at both ends. Check roll clearance by ex-
amining samples taken at a number of places un-
der the rolls. Another cause can be "Flooding" of
rolls or a concentrated load at one point. (Check
feeding device).
Rolls Will Not Take the Load - This can be caused
by several things:
1. The rolls are running too fast. The normal
speed of a 6" fast roll should be 980 RPM for a
3/16" diameter pellet.
2. Rolls running too slow. This is generally a
problem when the belts are slipping or the mo-
tor is overloaded. Here again, the spindle speed
should be checked.
3. Dull roll corrugations. It is important that roll
corrugations be maintained in good sharp operat-
ing conditions to assure proper cutting action and
a good production rate.
4. Poor feed distribution across the width of the
rolls.
43

45.
Cannot Make Fine Crumbs
1. On units with gear differentials, the gears can
be too large and thus prohibit the roll from clos-
ing tight enough. The steel gear should be recut
or replaced as a correction.
2. Corrugations could be worn, and the ad-
justing mechanism could be set so that the rolls
cannot get close enough. The recommended
correction is to recorrugate the rolls and readjust
them closer together. This has to be done care-
fully because the rolls cannot clash together or it
will ruin the corrugations.
Too Many Whole Pellets in the Crumbs
1. This is generally caused by a malfunction of
the bafﬂes at the ends of the rolls and can be
corrected by replacing the bafﬂes.
2. Occasionally, this condition exists because
there is too much clearance between the rolls
and the by-pass valve. One would anticipate
this problem after the rolls have been recorrugat-
ed one or more times.
Too Many Fines
1. This is generally due to poor pellet quality.
The remedy is to be sure the mash is properly
conditioned and that the pellets have been glued
together across the entire cross section with
moisture, steam and pressure. This is in contrast
to a poor pellet that has been burnt together on
the outside due to the effects of heat.
2. Overloading the crumbles rolls or concentrat-
ing feed at one spot can create additional ﬁnes.
3. The rolls could be dull and therefore, crushing
rather than cutting.
4. The pellets could be improperly dried and
therefore soft enough to fall apart under the ac-
tion of the crumbles roll.
H. PELLET SCREENING EQUIPMENT
The formed and cooled pellets or crumbs are
normally screened to remove oversize particles
and ﬁnes prior to shipment to the customer. The
degree of screening depends on local market
conditions and individual customer speciﬁcations.
The degree of sophistication required in the
sifting equipment is basically dependent on the
product mix run in a particular plant. A plant that
only produces pellets, and in particular, pellets
of one diameter, may get away with a single
screen type sifter. Conversely, the plant that
produces crumbs, 2 or 3 sizes of small pellets,
plus cubes, has an entirely different problem. The
options range from rapid screen change poten-
tial to a large involved screen with a number of
decks.
The location of a pellet screener depends on per-
sonal preference and/or plant layout. If located
in the basement, individual legs are required to
elevate the product, overs and ﬁnes to their ﬁnal
destination. The screen may also be located at
the extreme top of the feedmill, thereby allowing
for gravity conveying of the various portions.
The ﬁnal size and screen speciﬁcations for the
pellet screener are best determined through dis-
cussions with the various screen manufacturers.
The average capacity rates for the screening
operations in the pelleting system utilizing a rotat-
ing, gyratory-type incline screener are shown on
Table 1.
These rates vary considerably from installation to
installation depending on the percentage of ﬁnes
in a product and the speciﬁcations on the amount
of ﬁnes permitted in a product as it leaves the
screen.
Rate is also dependent on the proper feeding
to spread material across the entire width of the
unit.
Careful attention should be given to a screener’s
ability to keep the screens clean or free from
binding. Some sort of knocker mechanism is
required to provide the impact needed to move
ﬁnes through the screen wire. (See Plate #28).
The screener must be sized for the total ﬂow
of the product through the pellet mill, cooler
and crumbler, not the ﬁnished product rate
to the bin. In other words, allowance must be
made for the recirculating load.
44

46.
GENERAL CAPACITY DATA
PRODUCT OPERATION
SCREEN
SIZE CLOTHING
RATE
lbs/sq ft/hr
PELLETS SCALPING 5/8”-3/4” Space Cloth 1500
PELLETS FINES REMOVAL 2”-10” Tin Mill Screen 1500
CRUMBLES FINES REMOVAL 16”-18” Tin Mill Screen 1000
Table 1: General Capacity Data
Plate 29: Pellet Screener
The competitive situation in which a plant is
located has a signiﬁcant effect on the screening
layout. For instance, in a very competitive dairy
pellet market it may be necessary to go to a sec-
ond screening operation as the pellets are loaded
into bulk trucks, thereby taking out any ﬁnes
generated in the internal handling and binning of
the product.
Such conditions also exist in the competitive
range cube areas where most people ﬁnd it nec-
essary to locate a small screen directly over the
bulk loadout or bagging hopper. This is because
of the additional ﬁnes or chips formed in the in-
plant handling processes. For instance, the 100
foot drop to the bottom of an empty bulk bin really
does very little to keep down breakage.
Careful attention must be given to proper enclo-
sure of the screen and adequate aspiration
to prevent dusting. The day of the open dusting
pellet cleaner is rapidly drawing to a close. Such
dusty conditions are not only a housekeeping
problem but provide potential for explosion. It is
critical to ground the sifter properly to further
minimize potential for explosion.
Final pellet quality is again a signiﬁcant factor
in the screening equipment. It does no good to
remove all the ﬁnes after the cooler if the pellet is
of such poor quality that it cannot withstand the
rigors of in-plant handling.
Therefore, one needs to develop a standard
for a pellet’s ability to withstand the handling
it will receive, all the way to the ﬁnal custom-
er.
45

48.
I. Pellet Durability
Hardness testers for pellet quality have generally
given way to the durability testing. Durability
testing simulates the handling that pellets receive
in a normal feedmill situation. This testing mecha-
nism and test program were developed at Kan-
sas State University in the early 1960’s. This test
involves a prescribed agitation of pellets for
a predetermined time, measuring the percent-
age of ﬁnes generated.
The system involves a set of screens, Tyler type,
and a tumbling barrel as follows; per the A.S.A.E.
Standard: S269.1:
Section 6--DURABILITY
6.2 Pellets and Crumbles. The durability of
pellets and crumbles shall be determined by the
following procedure:
6.2.1 Device. Durability of pellets and crumbles
shall be determined by tumbling the test sample
for 10 mins. at R.P.H. in a dust tight enclosure.
The construction of this device is illustrated in
Plate = 29. The device is rotated about an axis
which is perpendicular to and centered in the
12” slides. A 2” x 9” plate is afﬁxed symmetrically
along one of its 9” slides to a diagonal of one 12”
x 12” side of the can. A door may be placed in
any side and should be dustproof. Projections,
such as rivets and screws, shall be kept to a mini-
mum and well rounded.
6.2.2 Screens. Fines shall be determined by
screening a sample on a wire sieve having open-
ings just smaller than the nominal pellet diam-
eter. Table 1 shows the recommended sieves for
crumbles and pellets of various diameters.
6.2.3 Test Procedure. A sample of pellets or
crumbles to be tested will be sieved on the ap-
propriate sieve to remove ﬁnes. If pellets of 0.5
in. diameter or larger are being tested, select
pellets which are between 1 1/4” and 1 1/2” in
length. Place a 1.102 lb. (500 gram) sample of
sieved pellets or crumbles in the tumbling can
device. After tumbling for 10 mins. the sample will
be removed, sieved, and the percent of the whole
pellets or crumbles calculated. Pellet and crumble
durability will be deﬁned as follows:
Durability =
Normally pellets will be tested immediately after
cooling. When the temperature of the pellets
falls within plus or minus 10° F of ambient,
they are considered cool. If tested at a later
time, the time in hours after cooling will be in-
dicated as a subscript of the durability. For ex-
ample, if the pellet durability tested 95 after a
four hour delay from the time of cooling, then the
results will be expressed as: (95)4. If pellets are
tested before cooling, there will be a signiﬁcant
weight loss caused by water evaporation, and the
apparent durability will be affected by this loss of
water vapor. The loss of water vapor must be de-
termined by making moisture content tests before
and after tumbling and compensating the ﬁnes
weight accordingly. When this procedure is fol-
lowed, the durability will be expressed as 1(95).
Weight of pellets or crumbles after tumbling x 100
Weight of pellets or crumbles before tumbling
47